The work of adding seed and fertilizer is an indispensable operation for seeding machinery unit. The research on quantitative relationship between seed or fertilizer adding parameters for seeding machinery unit and operational plot length is of great theoretical and practical significance to objectively analyze the regulars of seed and fertilizer adding parameters varying with plot length, and to improve the time utilization rate for seeding machinery unit. Based on the actual operation organization needs for the seeding machinery unit, the concept of the travel ratio of seed and fertilizer adding αw was put forward, the mathematic models of the distance between two adjacent seed or fertilizer adding positions, seed or fertilizer adding amount, seed or fertilizer adding times, and seed and fertilizer adding time for seeding machinery unit varying with the parameters of operational plot length were established under the conditions of three adding seed and fertilizer modes and with or without αw. Combined with experimental data and Matlab software, the various regulars of four seed and fertilizer adding parameters varying with operational plot length were determined when four typical seeding machinery units were operated with adding seed and fertilizer at one side of the plot. Regulars of four seed adding parameters of seeding machinery unit with or without αw all had a “two-ranges one-point” feature varying with plot length. When the plot area was 5hm2, the demarcation points between the corresponding plot length ranges with different seed adding parameters and with the same seed adding parameters, and the upper limit points of plot lengths that can be operated by four typical seeding machinery units with adding seed and fertilizer at one side of the plot were determined: the demarcation point of John Deere 7830 seeding machinery unit was 575m and the upper limit point was 1151m, the demarcation point of Valtra 171 seeding machinery unit was 438m and the upper limit point was 1031m, the demarcation point of Changfa 504 seeding machinery unit was 457m and the upper limit point was 1035m, and the demarcation point of the Huanghai 254 seeding machinery unit was 517m and the upper limit point was 1035m. The research results can improve the relevant application theory of agricultural machinery unit, provide theoretical and technical support for effectively improving time utilization rate and operational efficiency, and provide scientific basis for operation planning for seeding machinery unit and high-quality developing of agricultural mechanization.