Photovoltaic device based on van der Waals heterojunction provides an effective way to develop high-performance, low-power consumption, ultra-integrated micro photodetection system. In this paper, we construct an asymmetric Au/MoS ₂Schottky junction to realize a planar MoS ₂-based photovoltaic device. In order to further improve the photoelectric performance of the device, we design a structure covering MoO ₃on the surface of MoS ₂to construct the heterojunction. Owing to the absorption properties of MoO ₃in visible light and the excellent light transmittance of the ultra-thin two-dimensional structure, the electrons involved in conducting in MoS ₂material are increased. In most of previous reports, the preparation methods and performance improvement of MoS ₂/MoO ₃heterojunctions were the focus of research, but little attention was paid to exploring the influence of overlayer on devices. Therefore, in this work, we investigate the influence of overlayer thickness on device performance. With the help of atomic layer deposition (ALD) method to control the film thickness, each of the MoO ₃materials with thickness of 4 nm, 12 nm and 20 nm (deposition periods of 10, 30 and 50, respectively) is covered on the surface of a MoS ₂-based photodetector. The photoelectric performance enhancement effects of three groups of heterojunction photodetectors are compared with each other. The results show that the thinner the MoO ₃layer, the more significant the enhancement effect of heterojunction photodetectors is. This is mainly attributed to the fact that ultra-thin MoO ₃layer not only has visible light absorption, but also reduces the influence of the covering layer on the light absorption of MoS ₂, thus achieving a heterojunction system with high light absorption efficiency. In addition, the interfacial electric field of the heterojunction effectively promotes the separation of photogenerated carriers, and the thinner the MoO ₃coating layer, the weaker the effect of introducing the interfacial defects of the heterojunction is. Therefore, the dark current gain effect of the device is effectively suppressed, which is beneficial to improving the response speed and optical detectivity of the device. Comparing with pure MoS ₂photovoltaic photodetectors, the photoresponsivity of MoS ₂/MoO ₃heterojunction device in this paper is enhanced nearly 10 times. The device exhibits a high photoresponse of ~916.121 A/W, a detectivity of ~2.74×10 ¹¹Jones, and a fast response time of ~73 μs, showing that this design can effectively solve the low-responsiveness problem of planar photovoltaic device. In this study, for the first time, we construct a planar photovoltaic device based on MoS ₂/MoO ₃. By designing heterostructure and optimizing the thickness of the overlayer, the photoelectric performance of planar MoS ₂-based photovoltaic device is successfully improved, which provides a reference scheme for developing high-performance heterojunction photodetectors of MoS ₂/oxide materials in future.