The pulse width modulation (PWM) control for Lagrange systems driven by servo motors in stochastic environment is researched in this paper. Principles of PWM for stochastic nonlinear systems are proved, a continuous control bounded in probability of stochastic Lagrange system is designed, and the stochastic closed-loop nonlinear programming is modeled in the same form as that in the deterministic case. Sure boundedness of the control is required as a hard constraint, and the stochastic quadratic programming problem is presented for which an analytic solution is constructed and the expression of the optimal duty ratio is obtained. Finally, the main procedures of design and implementation of the stochastic PWM are summarized, and the effectiveness of the proposed scheme is demonstrated by a simulation of articulated manipulator.