To solve the problem of high frequency harmonics in the co-phase power supply system,San output filter LCL parameter design method which aims at the minimum THD (total harmonics distortion, THD) of grid side current is proposed.SFirst,Sfrom the overall perspective of locomotive current, grid side inductance,SandSoutput current from co-phase compensation device(CPD),Sthe paper deduces the relationship among THD of grid side current and RAF (ripple attenuation factor,SRAF) of CPD,Sthe grid side inductance,Sthe capacitance of LCL,Sthe total inductance of LCL,Sand the total inductance distribution ratio of LCL. Then,Sthe range of parameter value is determined by making CPD compensation current meet fast tracking ability,Sswitching ripple current meet the inhibition requirement,Sand LCL fundamental reactive power loss meet the restriction conditions.SThe minimum THD is the objective function and RAF is the constraint.SThe dividing rectangle algorithm is applied to solve the problem and find the parameter value of LCL with the minimum THD.S Finally, the accuracy of the parameter design is verified by MATLAB/simulink.