Application of 2 μm laser in lidar systems, remote sensing, requires more accurate wavelength. Based on the analysis of the quasi-three-level side pumped Tm:YAG laser system, the oscillating conditions of this laser were predicted from the point of pump threshold with taking into account reabsorption loss. The model accurately predictes that the laser central wavelength is switchable in the range of 2.0-2.1 μm by means of different laser parameters, such as crystal temperature, the crystal length, the transmission of the output coupler.