Abstract: The traditional dynamic light scattering method usually collects the lateral scattering to measure the size distribution of nano-particles. Because of the influence of multiple scattering, the particle size distribution of high-concentration samples cannot be accurately measured by lateral scattering. Aiming at the problem, the back scattering measurement method was studied, and the criterion of the optimal optical path of back scattering was proposed based on the experiment. At different sample concentrations, the polystyrene latex sphere particles with a nominal particle size of 110 nm and 220 nm were measured by lateral scattering method and back scattering method, respectively. The experimental results show that the back scattering method can self-adaptively adjust the optical path for high-concentration samples, and the measurement can be carried out at the optimal optical path to effectively obtain the particle size and the size distribution of high-concentration nano-particles, and the relative error of measurement results is 2.72%.