Abstract: Established a model of three-energy-level system from the laser rate equation theory，and then analysed the molecular dynamic processes and laser mechanism of the THz laser produced by optically pumped CH₃OH. According to the experimental parameters，the CO₂ lasers were used to pump CH₃OH to emit pulsed THz laser lines. When the 9P（36）line of CO₂ lasers pumps CH₃OH，the 118.8 μm line radiation was obtained，and then analysed the three-energy-level system's particles number and the characteristics of the output waveform at the pressure of 10 Pa and the temperature at 20 ℃，as well as the influence of buffer gas pressure ，intracavity temperature and output coupler reflectivity on laser output power. The results show that the theorectical computation can well reflect the process of generation of THz laser produced by optical pump.

Key words: rate equation, CH₃OH molecule, terahertz, optical pump