Image steganalysis is to discriminate innocent images and those suspected images with hidden messages. In this paper, we propose a unified Convolutional Neural Network (CNN) model for this task. In order to reliably detect modern steganographic algorithms, we design the proposed model from two aspects. For the first, different from existing CNN based steganalytic algorithms that use a predefined highpass kernel to suppress image content, we integrate the highpass filtering operation into the proposed network by building a content suppression subnetwork. For the second, we propose a novel sub-network to actively preserve the weak stego signal generated by secret messages based on residual learning, making the successive network capture the difference between cover images and stego images. Extensive experiments demonstrate that the proposed model can detect states-of-the-art steganography with much lower detection error rates than previous methods.