Abstract: For the speed servo system of the two-phase motor, analyzing its mechanical property and adjusting property while it is worked by amplitude control, factors to affect the two-phase motor dynamic characteristic are proposed, which are changes of its mechanical time constant and its gain factor. To prevent the dynamic characteristic of the system from being worse, as changes of its mechanical time constant and its gain factor, a simple following model reference adaptive control method can be used to improve the dynamic characteristic of the system and increase its controlling precision, by adjusting the model and the self-adaptive set. Computer simulation shows its availability finally.