Preparation of highly efficient and stable photocatalysts is crucial for the development of photocatalysis technology. In this study, the method of ultrasonic-assisted deposition and low-temperature calcination was used to prepare MoS₂/g-C₃N₄ S-type heterojunction photocatalyst (MGCD). Effects of the phase structure, micro-morphology, optical absorption performance, X-ray photoelectron spectroscopy, electrochemical AC impedance, and photocurrent of the materials on the photocatalytic activity were comprehensively investigated. The results show that, after ultrasonic-assisted deposition-calcination treatment, MoS₂ microspheres were broken, dispersed and combined on the surface of g-C₃N₄ nanosheets, and formed a kind of heterojunction. Under visible light, the degradation rate of 5%MGCD (with 5% MoS₂ addition) for Rhodamine B (RhB) reached 99% in 20 min, and still reach 95.2% when the sample was reused for 5 times, showing good photocatalytic performance and stability. Further analysis from the point of view of the formation of built-in electric field shows that the band bending caused by built-in electric field, coupled with MoS₂ and g-C₃N₄ in heterojunction, can effectively guide the directional migration of carriers, which can efficiently promote the separation of photogenerated carriers, thus improving the efficiency of photocatalytic reaction. Free radical capture experiment of heterojunction photocatalyst reveals that O₂^- and ·OH are the main active species in the catalytic degradation of RhB, followed by H⁺.