The gas state of the insulating gas significantly impacts the development and extent of the partial discharge (PD) in gas-insulated switchgear (GIS). Correlating these two factors allows for a more comprehensive diagnosis of insulation defects. At present, the ultrahigh-frequency (UHF) method has been widely used for PD detection in GIS. However, the measurements of the insulating gas's state in the field are primarily conducted offline or at gas valves, providing ineffective data for reference. To address this challenge, this article introduces a novel composite sensor capable of simultaneous UHF and gas state measurements. Building upon the disk-type UHF sensor structure, the composite sensor integrates a gas state sensing unit with the micro-electromechanical systems (MEMS) sensor as the core. A robust shielding shell is designed through finite element simulation to ensure the safety of the sensor, with the maximum induced voltage and electric field strength on the sensing unit measuring 0.28 V and 414 V/m, respectively. The average UHF measurement sensitivity of the composite sensor reaches 13.3 mm. In addition, the composite sensor exhibits low errors (±0.2 °C, ±3 kPa, and ± 10 μ L/L), high consistency and repeatability in temperature, pressure, and moisture measurement. Finally, the composite sensor has demonstrated its capability to simultaneously and safely detect PD signals and the gas state within GIS.