By testing on an oil ship, the acceleration-time signal of vibration on main engine was obtained. Based on interpolation operation of test signal and RD (Random Decrement) algorithm, the mode and other parameters of the signal were recognized. The impact of the sampling frequency and interpolation in algorithm upon the parameter was analyzed. Compared with FFT algorithm in test system, the RD result is more precise, and can be applied to the ship’s structure reliability design and fault diagnosis.