Phasor mearsurement unit (PMU) for distribution network is a new generation of distribution network automation terminal, which has the characteristics of rapidity, accuracy, reliability and expansibility. During the popularization and application process, it is urgent to solve the problems in installation, cost, precision of current sensor and limitation of broadband. In this paper, an ultra-fast synchronous phasor measurement method based on magnetoresistance effect is proposed. Current is calculated by a two-axis magnetic sensor chip and a single-axis magnetic sensor chip. With the obtained current signal, a half-wave time-frequency domain algorithm based on improved wavelet transform is proposed to calculate the synchronous phasor. In order to improve the computational efficiency and reliability of the algorithm, the step signal is recognized by improved wavelet transform. At the same time, a calibration is employed to shift between half-wave time domain algorithm and frequency domain algorithm. In order to further verify the effectiveness of the proposed algorithm, simulation tests are carried out on the simulation software. The results verify the effectiveness of the proposed algorithm.