A hybrid algorithm combining the characteristics of a conjugate gradient iteration method,a regularization equation and a Successive Over Relaxation (SOR) technique,namely,two-segment iterative method,is proposed to reverse efficiently the light intensity distribution into the particle size distribution in a laser particle sizer. The process of convergence can be speeded up and the smoothness of the solution can be enhanced by taking the conjugate gradient iteration as the pre-iteration of next SOR iteration. The solution obtained from the first step treatment is used as the initial solution for the second step. Then,a regularization method is used to correct to the original problem to obtain a stable regularized equation. Finally,the improved SOR method is taken to resolve the regularized equation.Experimental results show the two-segment method can achieve an accurate and stable final solution to this problem. This method is examined with the national standard particulate samples and the theoretical noisy signal,and the results indicate the method is able to distinguish multi-peak particle size distribution (PSD) and can smooth noisy signals.