Abstract: Acoustic emission signal needs high sample rate with large numbers of data and its energy is weak. According to these, the power spectral density analysis method basedon phase enhancement technique was studied. The signaltonoise ratio was increased with second FFT on the signal after highspeed sampling process without losing useful information. Simulation and practice diagnostic application verified the effectiveness of the method. Applied to detect the crack growth on the crank shoulder of septumpump in running condition, the weak acoustic emission signal due to crack growth was detected easily.