The channel characteristic of medium voltage power line is complex, in which the impedance mismatch will have an important impact on the quality of communication. The insertion loss is analyzed in the capacitive coupling device, which is caused by the non ideal parameters of the coupling capacitor and coupling transformer. A model of matching circuit is designed by using transfer power gain to describe the matching effect. Based on the structure of the classical passive bandpass filter and the parameter of non ideal element in coupling device, the component value of the matching network is adjusted by the nonlinear optimization algorithm. Finally, the average value of the transfer power gain reaches the maximum in the 50~500 kHz frequency range. Related examples show that the proposed method can effectively improve the transfer power gain of the whole frequency band and achieve a better matching effect. The structure of the matching circuit is simple and easy to implement. This work is supported by National Natural Science Foundation of China (No. 61204027) and Fundamental Research Funds for the Central Universities (No. 2015MS97).