A high-precision long-term online downhole temperature and pressure monitoring system based on optical fiber Bragg grating (FBG) and extrinsic Fabry-Pérot interferometer (EFPI) is developed in this paper. The system monitored the pressure through the change in the EFPI cavity length and the temperature through the change in the FBG wavelength, and simultaneously realized the temperature compensation of the pressure. Calibration results showed that the system has the pressure measurement range of 0.1‒42 MPa, the pressure sensitivity of 203.8 nm/MPa, the precision of ±0.05%F.S (F.S means full scale), and pressure resolution of 0.0007 MPa. The system is demonstrated in a production well in Zhuangxi, Shengli Oilfield, and the downhole temperature and pressure are recorded in real time for 5 months. The results show that the system can reflect the production status of oil wells and realize the online monitoring of important parameters, which has important guiding significance for improving oil and gas recovery.