In recent years, phototransistors based on two-dimensional semiconductors like transition metal di-chalcogenides including MoS2 have been studied extensively. Although the phototransistor based on monolayer MoS2 exhibits a high responsivity, its low carrier mobility also limits the response time of the photodetector to order of seconds. The stacking of two-dimensional semiconductors can form uniform van der Waals heterostructures with low trap defect states, and this is an effective way to improve the performance of two-dimensional photodetectors. In this work, MoS2/WSe2 vertical heterostructures were constructed via mechanical exfoliation and transferring method. The strong space charge region originating from the heterojunction can effectively separate the photo-generated carriers, therefore, the as-fabricated photodetectors have good photoelectric detection ability in self-powered mode. The responsivity and detectivity reach 2.12×103 A/W and 2.33×1011 Jones, respectively. Meanwhile, the response time of the heterojunction device greatly reduces to 40 ms. Such two dimensional heterojunction devices possess the advantages of simple fabrication method and good performance, and has a broad application prospect in the field of optoelectronics.