The change of voltage and reactive power in stable system caused by tap changing of different on-load tap changers (OLTC) has the nature of nearly linear superposition. Based on this, the strategy of changing the tap of OLTC is proposed in this paper, for satisfying the requirements of reactive power control. The step changes of voltage and reactive of all network nodes in power system caused by tap change of OLTC are calculated in initial power flow, which is defined as voltage effect per tap(VPT) and reactive effect per tap(QPT). The optimal strategy is obtained, by minimizing both taps changes(steps) and reactive power changes as the objective function, restraining the voltage of whole network buses to the control range, and optimizing the combination of transformer taps. The actual calculating example shows that the method is simple and practical.