为了对JPEG二次压缩后的图像进行有效检测，通过研究JPEG图像经二次压缩后所引起的DCT统计特性的变化及二次压缩前后的质量因子Ｑ１、Q２对二次压缩后图像的DCT统计所引起的变化的分析，提出了JPEG二次压缩的新的检测方法——抖动模式，并给出了满足DCT直方图抖动模式的充分必要条件。实验证明，这种新的检测方法是有效的。

After JPEG double compressed,the DCT statistic of JPEG image will be changed,which is the focus in this paper.The distributions of DCT coefficients are changed with the double compression factors: Q_1 and Q_2.The difference of the distributions is analyzed in this paper.A new method-dithering pattern is proposed to detect double JPEG compression.Meanwhile,the necessary and sufficient conditions about dithering pattern are obtained.Experimental results show that this method is effective.