Based on the principle of light extinction, two light extinction methods Independent model and dependent model method, for measuring the particle size distribution are persented in this paper. These methods have overcome the diffcu-ties of present light extinctions methods which can only measure mean diameter of particles and have limited measurable ranges and ambiguous measurement results, Moreover, the independent model light extinction method can measure the multi-peak polydispersion size distribution of particles. Numerical calculation and experimental research have shown the precision of the measurement results.