Through the analysis of the loss and threshold characteristics of a laser diode (LD), the threshold current density of LD with external feedback is calculated, according to the theory of multi-beam interference and the definition of equivalent reflection factor. When the length of external cavity approximates to the wavelength of LD, the obtained equation can be used to analyze the influence of the external cavity to the threshold current accurately. When the length of external cavity is much longer than the wavelength of LD, it is out of work. Because in the experiment the external cavity is much longer than the wavelength, the theory model is simplified by analysis to the injected power. The threshold drive current will be reduced obviously for the external feedback. The theoretical results agree well with the experimental findings.