Video transcoding is applied in the field of Internet video coding. When the original video is transcoded multiple times, only the distortion between the input video and the output video can be calculated and the distortion between the output video and the original video can not be learned. Here an algorithm for estimating the distortion between the output video and the original video was proposed to control the quality of the output program. Firstly, the superposition of distortion caused by multiple lossy transcoding was analyzed to derive the lower limit of total distortion. Then the probability method was exploited to make an estimation on the distortion between the original video and the final output video. Finally, the least square fitting was used to correct the estimation according to the prediction error. Experimental results demonstrate that the proposed algorithm can accurately estimate the distortion with the prediction error of 0.02dB, 0.05dB and 0.06dB for Y, U and V components on average respectively after correction.