A method of thermostatic control of LED as light source for differential optical absorption spectroscopy (DOAS) measurements is presented. A Peltier is used to control LED’s temperature and its working current is regulated through PID algorithm in order to keep LED’s temperature constant. Hardware design and software flow of thermostatic control system is demonstrated, and the thermostatic control effect and stability of LED’s spectrum are tested. The results show that the impacts of ambient temperature on LED temperature are effectively restrained and ±0.1℃ of control accuracy is obtained. Compared with the LED without thermostatic control under the same condition, the residual noise of the LED with thermostatic control is markedly lower, which also proves the availability of this method.