In the generator-motor system for hoisting in mine shaft exists a self-sustained oscillation caused by nonlinearity of excitation saturation of generator.This self-sustained oscillation must be controlled within a permissible range,otherwise it will leave hidden troubles for safety.This paper discussed the self-sustained oscillation with a given constant load and concluded that the value of the oscillation will reduce with the increase of given load,thus it can be made up to the optimum status just for overcoming static friction.