Heterojunctions based on low-dimensional materials can combine the superiorities of each component and realize novel properties. Herein, a mixed-dimensional heterojunction comprising multilayer WS2, CdS microwire, and few-layer WS2 has been demonstrated. The working mechanism and its application in a photodetector are investigated. The multilayer WS2 and CdS microwire are utilized to provide efficient light absorption, while the few-layer WS2 is utilized to passivate interfacial impurity scattering. In addition, based on the reasonable band alignment of the components, three built-in electric fields are formed, which efficiently separate the photo-generated carriers and enhance the photocurrent. In particular, the photo-generated electrons are trapped in CdS, while the photo-generated holes circulate in the external circuit, leading to a high photoconductivity gain. Motivated by these, we constructed a device that exhibits a photoresponsivity of ∼4.7 A W-1, a response/recovery time of 13.7/15.8 ms, and a detectivity of 3.4 × 1012 Jones, which are much better than the counterparts. All of these clearly demonstrate the importance of advanced device designs for realizing high performance optoelectronic devices.