Output energy of high power laser facility is mostly extracted from main amplifiers. Due to the circumstance complexity and diversity in paractice, it is difficult to build a precise microcosmic model in consideration of various factors. However, the existing predicting model of gain is imprecise, and no accurate model and corresponding experimental results are reported so far. Statistical methods are proposed to analyze the gain stability of main amplifiers in high power laser facility. The varying rules of the gain are put forward, including low frequency change, variation by the sequence number in one day and high frequency stability. Based on above-mentioned rules, the gain prediction model with weight factor is built and verified by SG-Ⅲ prototype operation data. The results show that the accuracy of gain prediction can be improved by one time in comparison with that of the old method, giving powerful directions for efficient operations in high power solid laser facility.