A U-shaped acrylic device is designed based on the principle of total reflection, and a method of liquid concentration detection based on dual optical path is proposed. By designing a U-shaped acrylic device and utilizing the evanescent wave total internal reflection effect at the liquid-acrylic interface, this study establishes a quantitative relationship between transmitted-light-intensity attenuation and the refractive index of opaque liquids. Real-time comparison between the experimental and control groups effectively eliminates the system error caused by the semiconductor lasers output-power fluctuations. The experimental results show that the absolute error of the methods measurements is less than 2%, the relative error is maintained at approximately 5%, and the coefficient of determination (R2) of the established concentration-relative-refractive-index calibration curve is 0.9967. Notably, the device achieves accuracy comparable to that of traditional methods used for transparent liquids, even when detecting opaque liquids. The system has the advantages of simple operation, an easily understandable principle, and low cost, providing a low-cost solution for the detection of liquid concentration in physics laboratories.