The external cavities of laser formed by independent movement reflectors in different directions is consided, the self-mixing interference (SMI) effect with multiway feedback is investigated. The basic theory of SMI with multiway feedback external cavity (MFEC) can be obtained by the theory of interference and the theory of laser SMI. Based on the external cavity structure with two feedback, a SMI model with two feedback external cavity structure is proposed. The expressions of the phase and output power in the MFEC are deduced and some simulations are made under different conditions. It is found that the output of the SMI system with two external cavities in different directions is sinusoidal or sawtooth waves, which is similar to the signal of SMI by low frequency sinusoidal phase modulation. The density of interference waves is uneven in one period and the shape of the interference wave is determined by the intensity of the external feedback. A SMI with two feedback external cavity experiment system is setup, and the experiment results testifies the validity of simulations.