The imaging technology of stimulated emission depletion (STED) fluorescence microscopy utilizes the nonlinear relationship between the fluorescence saturation and the excited state stimulated depletion. It implements 3D imaging and breaks the diffraction barrier of the far-field light microscopy by restricting depetion zone at a sub-diffraction spot. A model on the revision of the particle velocity equations was found to depict the temporal characteristics of the energy level population probability of fluorescent prob, and the oriterion called time verage depletion efficiency was defined. Gaussian function was used to simulate the two incident laser pulse, and the relationship between the depletion efficiency and the parameters, such as intensity, duration, delay time of both laser pulses, was obtained repsectively by numerical simulation, and the optimal value of these parameters was gained to improve the depletion efficiency is obtained. It is an effective approach to advance the system resolution.