We develop a novel ultrasonic sensor system using a fiber ring laser (FRL) to detect acoustic emissions. The sensor system incorporates two fiber Bragg gratings (FBGs) in the FRL cavity, a short and strong FBG as the sensing element and a long and weak FBG as the adapting element. The reflection spectra of both FBGs are matched such that the reflection peak of the long FBG is positioned at the linear slope of the short FBG's reflection spectrum. Ultrasonic waves impinging onto the FBGs are to modulate the FRL cavity loss, which leads to laser intensity variations that can be detected directly by photodetectors. The two FBGs are placed side-by-side in close proximity so that the sensor system is able to adapt to the ambient temperature drift. We demonstrate that the ultrasonic sensor system can operate normally within approximately 15°C temperature change. In addition, the performance of signal-to-noise ratios is investigated as a function of the FRL cavity loss. The proposed temperature-insensitive sensor system is attractive in practical applications where temperature change is unavoidable.