Using provincial panel data from 31 provinces in China's mainland regions from 2000 to 2020, this study calculates the provincial agricultural diesel technical efficiency based on the Data Envelopment Analysis Slack-Based Measure (DEA-SBM) model and the average agricultural diesel consumption per hectare for full mechanization. It constructs indicators to measure the level of farmland suitability for agricultural machinery operations (FSAM), analyzes the impact of FSAM on agricultural diesel technical efficiency using a fixed-effects model, and explores the specific mechanisms through which FSAM influences agricultural diesel technical efficiency using a mediation effect model. The research conclusions are as follows: 1) In general, provinces with lower levels of FSAM tend to have lower levels of agricultural mechanization and higher average agricultural diesel consumption per hectare for full mechanization, and vice versa. 2) Due to the continuous iteration and upgrading of agricultural machinery and the ongoing promotion of high standard farmland construction work, China's agricultural diesel technical efficiency has shown an upward trend from 2000 to 2020, with significant growth achieved after 2017, averaging an annual increase of 8.26% from 2017 to 2020, far exceeding the 1.78% increase from 2000 to 2016. 3) If the level of FSAM is improved from 0 to 1, it would enable an additional 10.06 hectares of farmland to be fully mechanized per ton of agricultural diesel, indicating a direct effect of 10.06 hm2/t on the enhancement of agricultural diesel technical efficiency. However, the upsizing of agricultural machinery equipment driven by higher FSAM levels increases the complexity of operations and the weight of machinery, leading to increased agricultural diesel consumption and reduced agricultural diesel technical efficiency, creating a masking effect. Consequently, the total effect of enhancing FSAM from 0 to 1 decrease from 10.06 hm2/t to 6.75 hm2/t. Based on these findings, the following policy recommendations are proposed: 1) Actively promote the transformation of farmland to enhance its suitability for agricultural machinery operations. As larger farmland plots with smaller slopes and better accessibility for tractor roads are more conducive to agricultural machinery operations, leading to higher agricultural diesel technical efficiency during operations and field transfers, efforts should be intensified to transform farmland to enhance its suitability. Current high-standard farmland construction focuses more on "ensuring harvests despite droughts and floods" while neglecting the adaptability of farmland to agricultural machinery operations, hindering the improvement of agricultural diesel technical efficiency. Therefore, in the construction of high-standard farmland, land consolidation measures such as "merging small plots into larger ones, transforming short plots into long plots, and reducing the slope of the land with high gradient" should be strengthened to facilitate efficient agricultural machinery operations and movements, thereby promoting the enhancement of agricultural diesel technical efficiency. 2) Develop lightweight agricultural machinery technologies. This study indicates that the large-scale power structure of agricultural machinery and equipment has a masking effect on the improvement of agricultural diesel technical efficiency by enhancing FSAM, impeding further improvements. The primary reason is that while agricultural machinery with high horsepower offers higher operational efficiency, the majority of these machines are constructed from steel. Consequently, the heavier the machinery, the greater its horsepower, leading to increased agricultural diesel consumption. It is advisable to minimize the overall weight of agricultural machinery while ensuring strength, safety, and operational effectiveness to reduce agricultural diesel consumption and carbon emissions. 3) Continuously develop green agricultural machinery equipment and technologies. Research and development investments in new energy agricultural machinery and biodiesel-powered agricultural machinery should be increased, and mature green agricultural machinery equipment and technologies should be promoted. Agricultural machinery purchase subsidy policies should appropriately favor these equipment and technologies to accelerate the pace of energy substitution and gradually resolve the conflict between agricultural mechanization development and the "dual carbon" targets at the root.