Photo-realistic image synthesis is an attracting idea for person re-identification (ReID) and data augmentation on human pose estimation. However, existing advances manipulating human image synthesis lack texture details for varying poses or appearances. This paper presents a person image synthesis Siamese generative adversarial network (PS2GAN), which re-synthesizes person image by changing the pose of that person to a given pose, modeled in a Siamese structure with image generative network and pair conditional discriminative networks in single-branch. For pose transfer, the proposed PS2GAN adopts Siamese structure consisting of two image generative networks and a novel contrastive-pose loss regularizing the generation process. Additionally, a nearest-neighbor loss computes the difference between fake and real images to make high-level information closer. Furthermore, the proposed PS2GAN is competitive to the state-of-the-art performance on Market-1501 and DeepFashion datasets via qualitatively and quantitatively comparing with prior works, and synthetic images of the PS2GAN can alleviate data insufficiency for person ReID.