Compared with Generalized Gaussian distribution (GGD), Cauchy distribution is superior to describe the statistical distribution of the Intra-coded DCT coefficients in H.264/AVC. For the bipolar additive watermark in H.264/AVC video stream, a Cauchy distribution based detection algorithm is proposed by ternary hypothesis testing. Experimental results show that the proposed approach can achieve more than 80% on average for the accuracy of watermark detection.