Abstract: It has become a main developing trend of the loom that the electronic let-off mechanism substitutes the mechanical one.This paper introduced the design of the driving system of a warp beam in which a step-motor was adopted as the let-off motor.By comparing DC,AC and step-motors and analyzing the driving characteristics of the warp beam,a conclusion was drawn that a higher performance to price ratio could be got when a step-motor was selected for the warp beam.The accelerating and decelerating of the step_motor during weaving and its driving principle were discussed,and the calculating method for parameters of the mechanical driving system was expounded with examples.