As the main factors affecting the accuracy of spectral measurement, temperature and pressure are the research focus of developing high-precision spectral measuring instrument. A high-precision temperature and pressure control system for spectral measuring instrument is developed. The system is designed and optimized from the aspects of instrument structure, circuit design and control algorithm, so the long-term temperature control accuracy of the system can reach 0.003 °C, and long-term pressure control accuracy can reach 5.34 Pa. Under the condition of pressure control but without temperature control, the measurement results of methane gas concentration with a volume fraction of 300×10-6 fluctuates by 12.06×10-6 and the deviation σ is 3.26×10-6. While under the condition of both temperature and pressure control, the fluctuation of the corresponding measurement results is 4.03×10-6 and the deviation σ is 0.57×10-6. Results show that the high-precision temperature and pressure control system can improve the measurement accuracy and stability of the spectral measuring instrument, and also verify the reliability and feasibility of the system. It is shown that the developed temperature and pressure control system for spectral measuring instrument has reached the experimental and production standards, which provides reference for the development of similar high-precision spectral measuring instrument.