This paper presents a dynamic regular groups steganalysis (DRS) algorithm to detect LSB steganography. This algorithm dynamically selects an appropriate mask for each image to reduce the initial bias, and estimates the LSB embedding message ratio by constructing equations with the statistics of regular groups in image. Experimental results show that this algorithm is more accurate and has a lower missing rate and false alarm rate than the conventional RS method and some other powerful steganalysis approaches present recently.