Abstract: A method for controlling a step motor of a scanner by using three sets of parameters is provided. The three sets of parameters include a pulse count number PCN, a count base CB, and a motor constant speed interval MCSI. The method includes the follow steps. First, generate a synchronous trigger signal in correspondence with each exposure trigger signal CVTR generated by an image sensing module of the scanner and clear a base accumulator BA. Then, add the count base CB to the base accumulator in correspondence with each reference pulse. Finally, generate an asynchronous trigger signal and clear the base accumulator when the base accumulator is greater than or equal to the pulse count number PCN. Since the speed of the step motor only relates to the pulse count number PCN and the count base CB, the step motor can be controlled by only setting these parameters.