The disclosure discloses a method and an apparatus for determining a state of power (SOP) of a battery system. With the method and apparatus, battery health state indexes determined according to the internal resistance and the current power of the battery system may serve as a calibration index, and the initial power parameters may be calibrated by using the calibration index. The internal resistance of the battery reflects the aging property of the battery system. Therefore, in the disclosure, the initial power parameters are calibrated according to the aging property of the battery system.