Inspired by the unique, thickness-dependent energy band structure of 2D materials, we study the electronic and optical properties of the photodetector based on the as-exfoliated lateral multilayer/monolayer MoS2 heterojunction. Good gate-tunable current-rectifying characteristics are observed with a rectification ratio of 103 at V gs = 10 V, which may offer an evidence on the existence of the heterojunction. Upon illumination from ultraviolet to visible light, the multilayer/monolayer MoS2 heterojunction shows outstanding photodetective performance, with a photoresponsivity of 103 A/W, a photosensitivity of 1.7 × 105 and a detectivity of 7 × 1010 Jones at 470 nm light illumination. Abnormal photoresponse under positive gate voltage is observed and analyzed, which indicates the important role of the heterojunction in the photocurrent generation process. We believe that these results contribute to a better understanding on the fundamental physics of band alignment for multilayer/monolayer MoS2 heterojunction and provide us a feasible solution for novel electronic and optoelectronic devices.