For the signal initial phase and frequency calculation of molded case circuit breaker instantaneous validation test, firstly this paper analyzes the calculation error of the initial phase of periodic signal based on FFT, then it proposes a method to calculate the initial phase based on Hilbert transformation, and at the same time carry on the measurement of frequency. The phase selection device is designed on the basis, and the design scheme of the hardware and software system is described. Finally the phase selection test shows that the method realizes the accurate measurement of the signal frequency and initial phase and the calculation error of signal initial phase can be effectively reduced. The designed device can accurately complete phase selection in instantaneous validation test.