The invention provides a power calculation method for a power battery system, a power calculation system for the power battery system and an electric vehicle. The method comprises the following steps of calculating SOC (State of Charge) of the power battery system of the electric vehicle; detecting a battery temperature of the power battery system; searching a table searching power corresponding to the SOC and the battery temperature from a pre-stored discharge power two-dimensional table as a theoretical maximal available power of the power battery system. The power calculation method, the power calculation system and the electric vehicle provided by the invention have the advantages that the calculation of the power battery system is simple and convenient, no complicated model is necessarily built, and wide application can be achieved.