The mechanized seeding operation is an important part in modern agricultural production, and the net operating time utilization rate of the seeder is an important indicator to impact the efficiency of seeder unit. According to machinery unit application principle, the new term of seasonal net time utilization rate was determined in this paper, each consumed time items of seeder unit was distinguished and relevant mathematic models were established based on mechanized seeding practice and sufficient survey, such as net operating time, turning time and time of adding fertilizer and seeds and so on. Based on this, three mathematical models of net operating time utilization rate of seeding machinery units were established according to different adding fertilizer and seed ways. The experiment scheme was designed, which uses the method of multi-point synchronous tracking measurement to test the time items in the seeding process without disturbing the normal seeding process. Data of each time items of 4 typical seeding machinery units were obtained during the experiments. The statistic principle of was used to distinguish and eliminate invalid experiment data. Based on the models established and effective tested data, Matlab 2012b was used to analysis the net operating time utilization rate of the 4 seeding machinery units changing with plot length and area under 3 different ways to add fertilizer and seed, the test results were expressed using Sigmaplot 12.5 software, and th quantitative correspondence between the different typical seeding machinery units and suitable operating plot area and length under the aimed net operating time utilization rate were determined. The results showed that when the aimed net operating time utilization rate of seeding machinery units is greater than 0.6, the John Deere 7 830 unit is suitable for the plots with areas no less than 6 hm2 and the plot length is 1 200-1 400 m. The Valtra 171 unit is suitable for the plots with area no less than 6 hm2 and the plot length is 1 000-1 200 m. The suitable plots for Changfa 504 unit are that area is no less than 1 hm2 and the plot length is 500-1 500 m. The plots with area no less than 0.3 hm2 and the plot length is 200-1 500 m are suitable for Huanghai 254 unit. The results can provide scientific accordance to select seeding machinery units for different plots.