In this paper, a cavity dumped Tm:YAG laser was obtained for the first time, whose output pulse width was constant at 54 ns. Maximum repetition rate was 200 kHz, and wavelength was 2013 nm. Its average output power was 595 mW. The laser cavity length was 208 mm with an inserted acousto-optic modulator. Pulses were coupled out of the lateral surface of the cavity when Radio Frequency was added into the modulator. And lens was used to compensate the thermal focal length of Tm:YAG crystal. Numerical calculation of pulse was done.