Abstract: Configuring a reactive power compensation capacitors as a resonant-free C-type filter is a common method to suppress reactive power compensation capacitors resonance. It can also be used to reduce the amplification of harmonic voltage caused by resonance and the power loss of the filter. However, the parameter selection range of resonant-free C-type filters is limited when the low frequency harmonic resonance needs to be suppressed, making it challenging to achieve low frequency harmonic resonance suppression economically. To address this issue, an optimal design method of a resonant-free C-type filter is proposed. Firstly, an improved C-type filter structure is designed to eliminate potential low frequency resonance points by incorporating a low frequency harmonic resonance suppression unit into the C-type filter. Next, a configuration model of the filter's element parameter is established, which takes into account the cost, power loss, the performance of resonant suppression and the selection range of element parameters. The optimal value of the improved C-type filter element parameters can be determined by the configuration model. The simulation and comparison results show that the filter designed by the method in this paper reduces the harmonic amplification ratio significantly and it has better performance of resonant suppression based on controlling the cost and power loss.