A tangential function is introduced to describe the nonlinear light-electriccurrent relation of a photo-electric diode. Based on the rate equations of semiconductorlaser and the nonlinear function equation mentioned above, we have established thetheory of bistable laser diode (BILD) and analyzed the major properties of BILD usingthis method. Finaly, the ways for improving the response feature of BILD are proposedaccording to the numerical analysis results.