Abstract: The moment of switching of the system for active thermoregulation of a resonator length of a frequency stabilized laser first switches on the feed of the laser tube and then continuously measuring the electric signal which depends on the laser resonator temperature. The signal is compared with a reference electric signal and at equalizing of both signals is switched the system for active thermoregulation of the resonator length of the frequency stabilized laser. The electric signal depending on laser resonator temperature is proportional to the rate of its temperature expansion.