The traditional FFT algorithm may cause spectrum leakage while calculating frequency and amplitude. The FFT algorithm is usually radix-2 FFT which can’t compute sequences that length is composite accurately. To solve these problems, an all phase-mixed radix FFT algorithm is proposed. All phase FFT, mixed radix FFT principle and the realization process of all phase-mixed FFT are also introduced. Through the actual test it is shown that all phase-mixed radix FFT algorithm outperforms traditional FFT algorithm in voltage and frequency measurement. Moreover, the algorithm is applied in the DSP for monitoring actual power grid as the core of the phase measurement device. The actual application result shows that the algorithm is accurate and reliable, and it has certain practical value upon the phase measurement.