A new measuring method and a measuring system for particle sizes based on the contrast of dynamic laser speckles were proposed by using a linear CCD camera. It was used to overcome the larger operation calculation and poor real time ability of the Dynamic Light Scattering (DLS) software and to solve the problem that the classic DLS could not measure the particles in high viscosity solution. Firstly, a model for the contrast of dynamic laser speckles was established based on the optical statistical theory and the classic DLS theory. Then, the relationship between the contrast of dynamic laser speckle and the autocorrelation function of dynamic light scattering was derived according to the Siegert formulation. Finally, the relationships between the contrast of dynamic laser speckle and the particle size in low and high concentrations were established. With the proposed method, aqueous latex spheres with diameter distribution of (490±20) nm and nano titanium dioxide powder glycerol solution with diameter distribution of 450-500 nm were measured under high and low concentrations. The results show that the proposed method not only has a small amount of computation, but also is able to measure the nanoparticles in the high viscosity solution. The measurement repeatability error of the new method is less than 2%, which satisfies the needs of national standard for dynamic light scattering.