The invention relates to a mounting assembly for an all-optical overvoltage sensor, and belongs to the technical field of voltage measurement. The mounting assembly comprises a sensor fixing housing, a transmission conductor expansion sleeve, a PVC hose, a polarization maintaining optical fiber patch cord disposed in the hose in a sleeving manner, an optical fiber insulator provided internally with a polarization maintaining optical fiber, and a connecting fitting. The transmission conductor expansion sleeve sleeves a transmission conductor fixedly. A sensor is mounted in the sensor fixing housing. The sensor fixing housing sleeves the transmission conductor expansion sleeve fixedly. Two ends of the PVC hose are respectively connected with the sensor fixing housing and the optical fiber insulator. Two ends of the polarization maintaining optical fiber patch cord in the PVC hose are respectively connected with a tail fiber of the sensor and the polarization maintaining optical fiber disposed in the optical fiber insulator. According to the invention, the mounting assembly can be used for fixing the sensor onto a transmission line for long term monitoring, non-electrical connection between a sensor measurement device and a power system is achieved, the safety of measurement is improved, and meanwhile, the impact of a non-measurement phase on the measurement is greatly reduced.