The changes of statistical features can be used to detect the existence of secret messages in images. In this paper, it is indicated that statistical distribution of the whole AC coefficients, such as Generalized Gaussian, Laplacian, Cauchy, etc. can be greatly affected by JPEG quantization process. Furthermore, a constraint condition concerned with quantization step is presented under which the coefficients of individual channel can be fit Laplacian distribution. Based on the constraint condition above, a new steganalytic method, which can achieve a high degree of detection reliability, is also proposed.