The invention provides a user month electricity consumption predicating method based on a seasonal index method. The method comprises the following steps that 1, electricity consumption historical data is obtained from an electricity consumption historical database by an electricity consumption historical data obtaining unit, and is stored in an electricity consumption historical data storing unit; 2, a seasonal index calculating unit works out a seasonal index Ij according to the electricity consumption historical data; 3, an electricity consumption predicting model building unit builds an electricity consumption predicting model by utilizing the seasonal index. The invention also provides a user month electricity consumption predicating system based on the seasonal index method.