Abstract: At present automatic mechanical transmission (AMT) has been widely used on automobiles, but it is seldom applied to tractors. China is a big country of producing and using tractors. Therefore, it is necessary to use AMT on tractors. This paper first analysed shift characteristic of tractors. Based on requisite characteristic field and supply characteristic field, the shift method of ATM for tractors was presented. Then, according to slipping rate mathematic model, engine power equation and some basic formulae in tractor theory, the relationship between driving force F_q and vehicle velocity v_a at each rack displacement of regulator was deduced and mathematic model of dynamic performance shift was developed. Connecting curve joints of F_q and v_a of two adjacent gears could get upshift curves. Finally, using tested data and computer programs, shift curves of Dongfanghong-1302R crawler-tractor were drawn. These provide theoretical basis for design and research of tractor AMT and do some useful attempts for enhancing technique content and international rival force of tractors in China.