The detection of oil-dissolved gases (such as CO, CH4 and H2) in transformers on-line is an important technology to predict and judge the operating status of transformers. Microstructure and gas sensing performance of Au-MoS2 microspheres synthesized by hydrothermal method were investigated in the present research. The results indicated Au-MoS2 based gas sensor has excellent response to CO, CH4 and H2 gases with maximum responses of 25.48, 16.75 and 8.48 at 30 ppm in their optimum working temperature (150 °C, 150 °C, 175 °C). In addition, First-principle calculation was carried out to expound the sensing mechanism in detail. The conclusions of this study suggest that Au-MoS2 monolayer exhibits a promising applicable future in sensor detecting of oil-dissolved gases in power transformer. The hierarchical flower-like Au-MoS2 microspheres were favoring synthesized using simple hydrothermal method and their microstructure are measured. Gas sensing performance and atomic level gas sensing mechanism of MoS2 microspheres to CO, CH4 and H2 was investigated. We found Au-MoS2 microspheres gas sensor has good gas sensing performances and Au-MoS2 monolayer exhibits excellent adsorption capacity to three target gases. Display omitted •The hierarchical flower-like Au-MoS2 microspheres were synthesized via simple hydrothermal method.•Au-MoS2 microspheres gas sensor shows good gas sensing properties to CO, H2 and CH4.•The excellent adsorption performances of Au-MoS2 monolayer to CO, H2 and CH4 were discussed in detail.•Theoretical studies have been applied to analyze the gas sensing mechanism of Au-MoS2 microspheres to CO, H2 and CH4.