An improved single-phase/three-phase phase-locked loop algorithm is proposed in this paper. This new algorithm treats the single-phase circumstance as a special unbalanced three-phase situation and then a three-phase phase-locked loop technology can be utilized and improved. In order to fast eliminate the coupled influence caused by positive sequence signal and negative sequence signal, a double frequency components extraction network is analyzed and built up for the double dq-synchronous reference frame PLL close-loop algorithm. This paper introduces the structure of this new algorithm and builds its transfer functions. The position variation tendency of the pole-zero points caused by sampling time and low-pass filter’s cut-off frequency is analyzed in detail and the analysis provides a direct guidance for PLL parameter design. In the end, simulation and experiment results verify the versatility, compatibility and effectiveness of this new phase-locked loop algorithm.