A new denoising method for the diesel engine transient speed signal was proposed, especially for the signal seriously corrupted by impulsive noises. This method takes two steps. The first step is to do impulsive noise removing. An improved differential rank impulse detector was used to do impulsive noise detection in this paper. After the impulsive noise was detected, it was replaced by the median value of the window it was in. Then the second step low pass filtering is taking on. Experiments show that the proposed method could remove the noise of the acquired diesel transient speed signals effectively, the denoised signal could reflect the real working conditions of the diesel engine.