In order to acquire the phase and amplitude of the positive sequence voltage under unbalanced grid voltage fast and quickly, a design scheme of three-phase phase locked loop based on inverse Park transform is proposed in this paper. Firstly, according to the analysis of the principle of positive and negative sequence voltage separation, a design scheme of orthogonal signal generator based on the inverse Park transform is proposed. Then, the detailed analysis of the orthogonal signal generator is carried out. In the meanwhile, the design approach of internal structure parameters is described in detail. Based on the proposed orthogonal signal generator, a three-phase PLL is designed. Finally, a PSCAD simulation model is built to test the performance of the proposed PLL. The simulation results show that the PLL has good adaptability and can quickly and accurately acquire the phase and amplitude of the positive sequence voltage.