The development of formaldehyde (HCHO) gas sensors with high sensitivity, good selectivity, and fast response at room temperature is an important research topic. The resonant microcantilever, with high sensitivity, easy batch manufacturing, and integration, generates sensing signals based on the relationship between the frequency and the mass addition of the adsorbed molecules and shows good application potential in HCHO detection. Herein, we report two species of MoS2 nanosheets (NSs) with 1T and 2H phases as sensitive materials to construct high-performance HCHO resonant cantilever sensors. The 2H-MoS2 gas sensors show better sensitivity toward HCHO compared to the 1T-MoS2 gas sensor. Specifically, the 2H-MoS2 sensor displayed a high sensitivity (Hz) of 13.6-1 ppm HCHO at room temperature, with high selectivity, low limit of detection (10 ppb), and good humidity resistance. The effect of phase structures on the sensing performance was studied by in situ characterizations, thermodynamic analysis, and density functional theory calculations. The good sensing ability could be attributed to the abundant active sites induced by the surface defects, suitable adsorption strength, and the outstanding thermodynamic performance of the 2H-MoS2. The combination of two-dimensional (2D) nanosheet and microcantilever sensors provides a new direction for developing a high-performance room-temperature gas sensor in the future.