According to 1951～2017 year climate factor indexes and precipitation data，this paper takes precipitation of the major flood season （April to June）as research subject，use linear trend method and Mann-Kendall test method to analyze precipitation trend in the major flood season of south Jiangxi，we conclude that precipitation in the major flood season of south Jiangxi is decreasing with a tendency rate of 17mm/10a in the long run，an evident downtrend has been happening since the end of 1980s.A census on the relationship between precipitation in the major flood season of south Jiangxi and pre climate factor indexes or pre precipitation has been conducted，then we established a long term trend prediction model for precipitation in the major flood season of south Jiangxi.We have verified that the model is reliable.This model can help us make a good preparation for flood control and disaster alleviation．