In this paper, a new design method of robust optimal proportional-integral (PI) controller with saturation characteristic is presented for different processes. Closed-loop feedback system is the nonlinear robust control system which can be equivalently transformed into robust linear switch system (RLSS) or robust autonomous switch system (RASS). The nonlinear robust optimal control problem can be equivalently transformed into nonlinear constraint optimization (NLCO) problem via the common Lyapunov functions (CLF) theorem, interval polynomial stable criterion and inference, and Lyapunov theorem. Therefore, robust optimal PI controller with saturation characteristic is obtained from solving the NLCO problem. Simulation results show the effectiveness and usefulness of the proposed design method.