A laser diode (LD) stack pumped solid-state laser (DPL) with different cooling conditions was reported. The thermal lens was calculated and compared under different cooling conditions. The influence of the thermal lens effect on the output energy fluctuation of the DPL was studied. The LD stacks peak power was 12 kW with 1-kHz repetition frequency and 20% duty ratio. In the examination with nothing to cool the solid-state heat capacity laser (SSHCL), the output power was 1 J at the beginning, after 1 s it fell down to 50%, and with the water cooling the output power hardly fell down.