To acquire the optimum value of the phase noise of signal source which is changing with the deviation of frequency from the carrier, area equivalent algorithmic is proposed to replace single-point method. The average power of phase noise is calculated by area equivalent algorithmic which utilize integration of area assuming the acquisition interval as a rectangle. Simulation was carried out using the average power of phase noise as only inference in the PCM/BPSK demodulation process, and the difference between the results of experiment in the same condition and simulation is no more than 0.7dB termed as Eb/N0 in the input, which is within 0.3dB compare to the demodulation loss acquired in white Gaussian noise. Experiments results prove that it is a practical method in test.