Abstract: China has a vast acreage of cropland for plastic-shed vegetable production, in which carbon sequestration and greenhouse gas emission reduction achieved via various optimized farming practices are of great significance to the green agricultural development of the nation and of the world. In order to clarify the effects of fertilizer application and straw returning on soil organic carbon (SOC) in plastic-shed vegetable production in northern China, a long-term field experiment initiated in 2004 was conducted in Shouguang plastic-shed vegetable base. In this experiment, 6 treatments were set up to study the change rule of soil organic carbon under different fertilization and straw returning measures by measuring and analyzing soil organic carbon content, storage, carbon conversion efficiency of organic materials, and carbon fixation rate of organic matter. The treatment included control (CK), organic fertilizer + straw return (MS), organic fertilizer + optimized nitrogen (N) fertilization (N1M), organic fertilizer + conventional N fertilization (N2M), organic fertilizer + optimized N fertilization combined with straw return (N1MS), and organic fertilizer + conventional N fertilization combined with straw return (N2MS). The SOC contents and stocks at 0-30, >30-60, and >60-90 cm layers for pre- and post-experiment were monitored. The results showed that N1MS treatment could significantly increase soil organic carbon content in 30~60 cm soil layer (P<0.05), which was 40.5% higher than that of N1M treatment, while under N2M treatment, the effect of straw returning on SOC increasing was not significant. N2M treatment significantly increased the soil organic carbon content by 47.0% for 0-30 cm and 17.3% for >60-90 cm layers, compared with that of optimized N fertilization (N1M). With the application of organic fertilization and straw returning, SOC contents in the 0-30 cm layer increased with the increase of N fertilization application, and the increase of SOC content was at the order of N2MS (106.5%) > N1MS (64.2%) > MS (39.9%). In the 0-90 cm soil layer, the highest SOC stock was 115.5 t/hm2 for N2MS in 2015, followed by 110.5, 104.0, 101.7, and 98.0 t/hm2 for N2M, N1MS, MS, and N1M treatments, respectively, and all were significantly higher than those without organic fertilizer (CK). the SOC sequestration rate of N2MS in 0-30 and 0-90 cm layers was 1.85 and 3.74 t/(hm2?a) respectively, which indicated that for intensive cropping like plastic-shed vegetable production, deep-soil carbon stock should be accounted for national carbon quantification. In the 0-30 cm soil layer, the organic material carbon conversion efficiency of the scenario of conventional N fertilization was significantly higher than that of optimized N fertilization, in which N2M was 204.9% higher than that of N1M and N2MS was 130.3% higher than that of N1MS, which indicated the increase of N fertilization rate was helpful to improve the conversion efficiency of organic materials to SOC. In the 0-30?cm soil layer, the carbon conversion efficiency of organic material in N1M treatment was 35.5% higher than that in N1MS, and in N2M it was 79.4% higher than that in N2MS. In plastic-shed vegetable production, straw returning reduced the conversion efficiency of organic matter to organic carbon, and fertilizer nitrogen and straw returning have no positive interaction on organic carbon accumulation, due to the high level of organic fertilizer. In China, the carbon sequestration efficiency and carbon conversion efficiency of organic fertilizer and straw in plastic-shed vegetable production should be further improved by optimizing chemical and organic fertilizer inputs.