Abstract: The structure of double-crystal four-end pumped Tm:YLF laser was designed and studied. By using steady-state thermal analysis module in the Ansys software, the temperature distributions of the two crystal-clamps and cooling plate were calculated with the heat exchange coefficients of the cooling plate channels given as 1 000 W/(m2℃ ), 4 000 W/(m2℃ ), 8 000 W/(m2℃ ) and 15 000 W/(m2℃ ). Meanwhile, the temperature distributions when the cooling-plate was assumed as a TEC cooler with temperatures of 18℃ and 20℃ were calculated, respectively. According to the calculated results, the cooling effect when the cooling-plate was given as medium convection intensity or higher would be equal to the result when taking the cooling-plate as TEC with temperature of 18℃ and 20℃. According to the simulation results, a U-shape Tm:YLF laser was designed pumped by fiber coupled laser diode. The output lasers with powers of 25.9 W and 46 W were achieved from single crystal and double crystals when the Tm:YLF crystal clamps and cooling plate were directly cooled by 16℃ cold water, corresponding to slope efficiencies of 40.7% and 37.1%, respectively. No over high temperature was observed in the whole experiment. The experiment results indicate that the designed direct conduction cooling system can effectively remove the heat load in single-crystal dual-end and double-crystal four-end pumped Tm: YLF laser.