In dynamic equivalents of power systems， if the excitation systems of the coherent generators are of the same type， the equivalent excitation system has the same type and the same model， and the equivalent parameters could be obtained with the weighted summation. However， the excitation systems of the coherent generators may be different. For this case， the uniform stand transfer function as the model of equivalent excitation system are proposed， and the parameters of the equivalent excitation systems are estimated by piecewise linear polynomial function（PLPF） method. The simulation results show that the frequent and time domain characteristics are very proximal before and after excitation system aggregation. This proves the effectiveness and adaptability of the proposed method.