In order to optimize the capacity of co-phase compensation device in combined co-phase power supply system, the relationship among relative capacity (β1) on co-phase compensation device and traction transformer, negative sequence and power factor is analyzed. Based on structure analysis of combined co-phase power supply system, with the usage of principle of transformer connection, the compensation principle of traction substation and symmetrical components theory, the relationship among traction transformer capacity, co-phase compensation device capacity, system short circuit capacity and load capacity is obtained. And then a series of values of β1 on complete compensation and satisfying compensation are analyzed respectively. The search result indicates as follows. The value of β1 is 1 when completely compensated. High speed railway doesn’t need reactive compensation when satisfying compensation on reactive power because its power factor is nearly 1, but ordinary speed railway set reactive power compensation factor according to power factor of load. Both of them need determine the values of β1 according to the short circuit and load capacity. Eventually simulation results by Matlab verify the validity of analytical calculation. This work is supported by Gansu Province Science and Technology Plan (No. 1504WKCA016), Tutors of Project in Gansu Province (No. 2015A-055), and National Natural Science Foundation of China (No. 51367010).