PV array P-U characteristic curve in partial shadow state has multiple peaks. To solve this problem, a three-step global maximum power point tracking (GMPPT) strategy based on search-calculation-determination is proposed. The improved disturbance observation method is used to search the first peak point in reverse direction, and the search area is reduced by calculation and judgment. The multi-peak value search can be indirectly transformed into a single peak value search, which overcomes the problem of falling into local extremum in the search process. Through the establishment of three kinds of shadow state models, this method is compared with the existing maximum power point tracking (MPPT) algorithm, and the corresponding experiments are established. The simulation and experimental results show that the method can stably realize the GMPPT in the state of shadow and noshadow, and effectively improve the tracking accuracy and speed.