The MoS2/graphene composites were prepared by hydrothermal method. Then the composites were fabricated to gas sensors for exploring the sensing characteristics of mercury vapor. When the molar ratio of graphene and MoS2 was 0.5%, the composites has the best gas sensor response. The concentration in 2.18-126.18 mg/m3 of mercury vapor was tested, and the sensor show excellent linearity. When the concentration of mercury vapor is 29.48 mg/m3, the response time of the sensor is 8.5 min and the recovery time is 9.0 min. The sensor has excellent repeatability, stability and selectivity. Other common gases in coal-fired power plant flue gas such as H2S, NH3 and NO2 has no obvious response, and the sensor can be used for practical detection.