According to the characteristics of signal modulation and transmission, a heterodyne interference system composed of two acousto-optic modulators is designed and built. The two-way light intensity modulation avoids the effects of diffraction level mixing and frequency shift caused by acousto-optical modulation. The beat frequency signal formed by the interference of diffracted light from two +1 stages is finally output by using dual modulation. The inter-stage interference is eliminated. At the same time, the system has good collinearity and symmetry, which greatly reduces the influence of the external environment on the measurement and improves the detection precision. In this paper, the phase change information caused by the change of refractive index is detected and processed. The phase characteristics of the system are studied by measuring solutions with different percentage concentrations, and a new solution concentration detection scheme is proposed. The influence of laser drift and modulation intensity on the measurement system is analyzed as sell. The experimental results show that the detection system can effectively detect the concentration of the solution by processing the signal phase difference change information. In the range of 0.1%~0.5%, the maximum relative error is about 8.0%. It provides a reference for the design and fabrication of a miniaturized optical external diffrential interference solution concentration measurment system.