LCL filter has been widely used for harmonic suppression in power system converter, especially at high power conditions. For instance, in the 27.5 kV locomotive co-phase traction power, the railway power quality conditioner is a backto- back converter, and LCL filter is preferred to suppress the harmonics injected. In this paper, the design procedure of LCL filter for harmonic suppression in co-phase railway power quality conditioner is being analyzed and proposed. The influence of LCL filter parameter on its performance is being derived mathematically and discussed. Besides normal discussion of current harmonic suppression, voltage harmonic suppression performance of LCL filter is also investigated. Finally, the LCL filter design is being verified via PSCAD simulation. This paper provides a systematic design procedure for LCL parameter design, which is derived based on mathematical analysis.