A new method is proposed to measure the thermal focal length of laser crystal. Based on the experimental results, a laser diode (LD) side-pumped, flat-flat cavity regenerative amplifier is developed. By injecting a small signal seed light with wavelength of 1064 nm, pulse width of 2 ns, repetition rate of 300 Hz and pulse energy of 300 pJ, the output TEM00 laser with maximum pulse energy of 1.5 mJ, pulse width of 2 ns is obtained. The corresponding amplification ratio is 5×106, and the output energy stability is 1.08% [root-mean-square (RMS)] with the square-pulse distortion of 1.10.