Characteristics and laws of the variation of self-mixing interference signal of laser diode along with the increase of external-cavity length are analyzed theoretically. The self-mixing interference signal attenuating exponentially along with increase of external-cavity length is deduced from the composite cavity interference function. The attenuation coefficient is direct ratio to imaginary part of light frequency. A group of transcendental equations for imaginary part and real part of light frequency are also presented. By solving the equations, at external-cavity length more than a critical value, along with the increase of external-cavity length, the imaginary part is inversely proportional to attenuation, so the self-mixing interference signal attenuates very slowly. When external-cavity length is less than the critical value, feedback light increases the imaginary part, moreover, the imaginary part oscillates along with external-cavity length. The self-mixing interference signal appears as a complicated state. The greater the intensity of the external cavity feedback or the single-mode line-width of laser is, the smaller the critical value is. Analysis of the laser diode self-mixing interference also needs to consider the effect of feedback light on the imaginary part.