The AC filter is the important device of the HVDC system, and the AC system harmonic impedance is the important input condition of the AC filter design, which directly affects the equipment cost. In order to solve the problem of AC filter design excessively difficult due to the sector or circle harmonic impedance. The characteristics of different harmonic impedance boundaries are compared, and polygonal harmonic impedance construction method is proposed. The calculation method of the maximum parallel impedance based on polygonal harmonic impedance is proposed. Finally, a design example is used to compare the AC filter performance under different harmonic impedance shapes. It is verified that the proposed polygonal harmonic impedance can effectively reduce the design difficulty of the AC filter and improve the filtering effect.