MoS2/RGO heterojunction composite was fabricated by a facile hydrothermal method. Ultra-thin MoS2 nanosheets were vertically grown on RGO in situ with C-O- functional group at the edges and plane of RGO as nucleation sites. This strong interfacial interaction between MoS2 and RGO not only can increase the active sites of MoS2 but also can alleviate the contact resistance between MoS2 and RGO. Photodegradation of rhodamine (RhB) was adopted to evaluate the photodegradation ability of the products. Photocatalytic degradation performance of MoS2/RGO composites with different composite ratios was also investigated. The degradation rate of 15 mg 7.5 wt% MoS2/RGO (RGO:MoS2) composite for 50 mL RhB solution with a concentration of 20 mg/L within 60 min can reach 96.6%, showing the highest photodegradation activity. The transfer and transport mechanism of photogenerated charges at composite interfaces are discussed in detail to explain the enhanced photocatalytic performance. After five cycles of catalyzing, the photocatalytic performance of MoS2/RGO composite was not significantly lower.