Based on Fabry-Perot cavity’s theories and efficiency refractive index, the optical bistability of a nonlinear microcavity is studied, and the corresponding mathematical formula are deduced. The theoretical curves are well fitted with those of numerical simulation in other papers. The investigation shows that appropriate frequency-shifting should be preset on the incident light in order to produce bistability for a nonlinear system. If the system is composed of focusing nonlinear medium, the frequency must be red-shifted, otherwise it must be blue-shifted. The magnitude of critical shifting is 0.866 times of the width of microcavity resonance mode.