Aimed at the problems of DC bias and frequency variation in a weak grid, a modified inverse Park transform phase-locked loop (IPT-PLL) technology suitable for single-phase grid-connected inverters is proposed. First, the α component after Park transform is used as a reference voltage in the phase detector to solve the problem of DC bias in grid voltage, and an orthogonal component is constructed by the method of 1/4 fundamental periodic delay. Second, the fractional-order delay is approximated by Lagrange interpolation polynomial to reduce the calculation error of delay caused by frequency variation, and the design method for PI regulator is theoretically analyzed. Finally, experimental results show that the modified IPT-PLL proposed has a strong frequency adaptivity, and it can significantly suppress the interference of DC bias in grid voltage. In addition, its dynamic and static performances are satisfying.