Experimental and theoretical studies of a laser diode (LD) array pumped solid-state heat capacity laser (SSHCL) were reported. The thermal lens effect of heat capacity slab Nd:YAG laser under side pumped condition was calculated, and the influence of the thermal lens effect on the output energy fluctuation of the SSHCL was studied. In experiment, slab crystal size is 57 mm×40 mm×4 mm, peak pumped power of the LD array is 12 kW with 1 kHz repetitive frequency and 20% duty ratio. In order to obtain high laser gain, the pump beams were focused as small as 15 mm×57 mm spot in profile. In the experimental examination, the output of single pulse energy was 1 J at the beginning, after 1 s operation it fell down to 50%.