Ratio of self-mixing interference amplitude and static light intensity from a laser diode is only a few percent based on experimental observation. Feedback light decreases threshold current of laser diode and increases the static light intensity. The feedback light makes laser diode operate over the actual threshold current. The output intensity of laser diode is calculated with a three-mirror Fabry-Pérot cavity system. An equation between light frequency and feedback light is found from the main maximum condition of interference function. Signal waveform of self-mixing interference and ratio of the signal amplitude with the static light intensity are calculated, and the results are coincident with that measured experimentally. There are 15 resonance modes of complex cavity in the case of τrC/τi=22.8 at the same time. The variations of these resonance modes with external cavity length are less than 2.6×10-2 cm-1 and far less than 3.1 cm-1resonance mode width of Fabry-Pérot cavity. The variation of the light frequency is within the resonance mode width of inherent cavity.