Abstract: In order to discuss approximation degree of calculating equivalent reflectivity and other parameters of the fiber Bragg grating external cavity laser diode by using two equivalent cavity models of the three cavity mirrors approximation or the coupled cavity approximation and choose the best one, the threshold gain characteristic curves of two approximation models by using apodized fiber Bragg grating and normal fiber Bragg grating were gotten after numerical caculation and simulation. Numerical analysis results show that three cavity mirrors approximate model is more in line with the actual situation when using the apodized fiber Bragg grating. This conclusion has the reference in analysing and designing single longitudinal mode fiber grating external cavity semiconductor laser.