Abstract: The wide application of power electronic equipment has brought about new problems of stability and reliability in power system. It is urgent to use high-precision real-time measurement and sensing technology to provide the data base. As one of the most effective dynamic monitoring devices, the measurement accuracy of phasor measurement units（PMUs） is very important. A reference phasor calculation method for PMU calibrator is presented, which provides reference measurements for PMU test. The characteristics of typical static and dynamic signals in power system are analyzed, and a general signal fitting model for a fundamental phasor is established, which can characterize the time-varying characteristics of phasor parameters besides step test signals. An iterative method for solving the amplitude, phase angle and frequency of the phasor based on nonlinear fitting is proposed, and a calculation method for the rate of change of frequency（ROCOF） based on the least squares method is proposed according to the measured frequency. Furthermore, the influence of the iterative initial value, observation window length, and fitting order on the measurement accuracy and the setting method are analyzed. The simulation and experimental results show that the accuracy of the proposed method is at least one order of magnitude higher than the requirements defined in PMU standards. Therefore, the proposed estimation method can be applied to the test and calibration of PMUs.