Recently, the vector similarity measurement (VSM) algorithm has been introduced to analyze particle size distribution (PSD) of the forward scattering. However, the retrieval result is not ideal for multi-modal distribution particle systems, especially for the particle distributions of more than three peaks. To satisfy the need of predicting multi-peak distribution particle and enhancing common applicability, a modified iterative algorithm is proposed based on vector similarity measurement, and it is applied in retrieval calculation of PSD function. Simulated results and experimental researches show that the algorithm can also get the reasonable PSD with high measurement error.