An all fiber-optic temperature sensor based on the temperature dependence of radiation-induced attenuation (RIA) of Ge/P co-doped optical fiber at 850 nm was proposed. The principle of the sensor was interpreted by the temperature dependence of color center absorption. Characteristics of the temperature sensor, such as dynamic range, sensitivity, linearity, and repeatability, were tested and analyzed. The dynamic range of the sensor was-40°C to 60°C, and the temperature sensibility was 0.1146dB/°C. The nonlinearity error and the repeatability error were respectively ±2.75% and 1.06%.