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/MoS₂ Schottky junction to realize a planar MoS₂-based photovoltaic device. In order to further improve the photoelectric performance of the device, we design a structure covering MoO₃ on the surface of MoS₂ to construct the heterojunction. Owing to the absorption properties of MoO₃ in visible light and the excellent light transmittance of the ultra-thin two-dimensional structure, the electrons involved in conducting in MoS₂ material are increased. In most of previous reports, the preparation methods and performance improvement of MoS₂/MoO₃ heterojunctions 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 MoO₃ materials 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 MoS₂-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 MoO₃ layer, the more significant the enhancement effect of heterojunction photodetectors is. This is mainly attributed to the fact that ultra-thin MoO₃ layer not only has visible light absorption, but also reduces the influence of the covering layer on the light absorption of MoS₂, 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 MoO₃ coating 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 MoS₂ photovoltaic photodetectors, the photoresponsivity of MoS₂/MoO₃ heterojunction device in this paper is enhanced nearly 10 times. The device exhibits a high photoresponse of ~916.121 A/W, a detectivity of ~2.74×10¹¹ Jones, 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 MoS₂/MoO₃. By designing heterostructure and optimizing the thickness of the overlayer, the photoelectric performance of planar MoS₂-based photovoltaic device is successfully improved, which provides a reference scheme for developing high-performance heterojunction photodetectors of MoS₂/oxide materials in future.