The invention provides a flood season division risk quantitative evaluation method. The flood season division risk quantitative evaluation method is characterized by comprising the following steps of 1, sampling the largest flood of a main flood season and a non-flood season in a history sequence; 2, selecting a random variable marginal probability distribution model; 3, deducing a largest flood expression mode of two divided seasons, including a main flood season and a non-main-flood season, which possibly occurs in the whole history sequence time length and a corresponding accumulative probability density function; 4, quantifying the risk that the occurrence of a flood in the non-main-flood season exceeds that in the main flood season, and solving a specific expression formula. The flood season division risk quantitative evaluation method can be widely applied to water resource management practices, the flood risk of all season division schemes can be quantitatively evaluated, and therefore scientific technical support is provided for flood risk evaluation and flood resourcefulness of a river flood season division scheme.