A system of fiber Bragg grating (FBG) temperature sensing demodulation based on light power detection is proposed in this paper. Compared with the traditional demodulation method based on wavelength scanning, light power detection is more direct and avoids the use of spectrometer. Moreover, the light power in the system is converted into the electrical signal by the receiver optical subassembly (ROSA) and converted to the digital signal. The micro controller unit (MCU) processes the digital signal to realize the real-time temperature monitoring, which avoids the use of optical power meter (OPM). With the advantages of simple structure and low cost, the system is portable and practical. The experimental results show that the linearity coefficients R-square between light power and the sampling voltage are 0.999 08 and 0.998 93 in the temperature range from 10 °C to 85 °C, respectively. According to the results, the proved sensor has a repeatability error of 1%, a linearity error of 1.35%, and a hysteresis error of 0.7%, which indicates that the system is of high stability and high precision. The experimental results are consistent with the theory, which verifies the system's feasibility.