The ＇＇dependent model＇＇ is one of the widely used algorithms in light extinction particle sizing. But how to determine the upper and lower limits of the integral to match the size range of measured particles is still a problem with this algorithm. By analysing the Rosin-Rammler (R-R) distribution function, the authors find that there is a relationship between integral range and the distribution parameter k of the R-R function. The integral range depends entirely on the value of k. The integral ranges corresponding to k are presented in this paper. Based on these data, a self-adaptive program was written to process the measured data. Numerical simulation and experiments were performed to test this algorithm, and the results are presented in this paper.