When the grid frequency varies, the ratio of system sampling frequency to grid frequency may be a fraction. The resonant frequencies of Repetitive Control (RC) will deviate from real grid frequencies, and the harmonic suppression capability of system will be significantly reduced. Therefore, a Frequency Adaptive PIRC (FA-PIRC) scheme based on FIR filter is proposed. The fractional delay is realized by adjusting the coefficients of FIR filter online, that the resonant frequency of RC is approximated to grid frequency, thus realizing the adaptation of system to grid frequency variation. Finally, simulation and experiments verify the effectiveness of the proposed control strategy. This work is supported by National Natural Science Foundation of China (No. 61673404) and Key Science and Technology Project of Henan Province (No. 152102210155).