The novel sensing materials of rGO decorated CdS/CdO heterojunction have been synthesized successfully for the first time by a facile one-step hydrothermal method combining with calcination treatment. The structure, morphology, composition, and specific surface of the composite were characterized by XRD, SEM, XPS spectra and BET analysis, respectively. The gas sensing properties to 2 ppm of NO2 were examined. The results show that the 4.0 wt rGO/CdS/CdO composite not only exhibits 9 times higher response and 4.75 times lower detection limit than that of pristine CdS, but also owns rapid response and recovery, indicating the composite is a promising novel sensing material for detecting of low-concentration NO2. The enhancement mechanism was also discussed in detail, which benefits from the formation of the heterojunctions between CdS and CdO and the decoration of rGO. rGO also as an efficient electron mediator increases gas adsorption and accelerates electron transfer of the composite.