A computational method of excitation inductance of power transformer needs current of all windings. However the excitation inductance of power transformer with Y-Delta connection cannot be calculated directly using any existed method because the winding currents on the Delta side cannot be measured based on existed TA. An algorithm to calculate the excitation inductance of power transformer with Y-Delta connection is proposed in the paper. Based on the equivalent circuit equation， the excitation inductance of power transformer with Y-Delta connection is calculated by using the line currents on the Delta side directly and by eliminating the internal currents of windings on the Delta side. The algorithm accuracy is verified by EMTP simulation. The result shows that the excitation inductance calculated using the algorithm proposed can reflect the transformer and its iron core status accurately， effectively and sensitively. It can be used as the criterion of transformer protection and has good project application prospect in the field of the transformer protection.