For the technology of phase selection of instantaneous calibration test for molded case circuit breaker, this paper proposes the technology of phase selection based on Hilbert transformation. The basic principle and algorithm of it are analyzed, and this method can dynamically measure the practical frequency, phase and power factor angle of voltage and current; Compared with the traditional FFT calculation method, parameter calculation accuracy is not sensitive to the grid frequency fluctuation and the testing effect is good under amplitude fluctuation or high SNR condition; By adding-window functions to the sampling signal, it further improves the calculation accuracy of phase selection parameters. Finally, it provides the design scheme of phase selection equipment based on PIC microcontroller.