To diagnose faults in engine accurately, technology on unsteady vibration signal acquisition(UVSA) from engine is studied. Measure principal, software and hardware composition of UVSA system at fixed rotary-speed is discussed and method of frequency bond energy accumulation(FBEA) is introduced. Based on short term Fourier transform (STFT), the repeatability and stability of acquired unsteady signal is analyzed on two models of vehicle (EQ2102 and SX2190) by using FBEA. Experimental results show that UVSA instrument can accurately measure the vibration signal containing specific fault characters during the acceleration or deceleration process, and it has better repeatability and stability.