A highly stable, laser diode (LD)-pumped, all-solid-state Nd:YLF regenerative amplifier applied to amplify a nanosecond laser pulse is designed and realized. The maximum output energy and the best stability of the regenerative amplifier can be obtained when it operates in a saturated mode for all pulse duration and temporal profiles. In addition to the excellent stability, the output laser pulse fluctuation caused by the injected pulse energy fluctuation is inhibited. Extra post-pulse can decrease the temporal pulse profile distortion caused by the gain saturation effect. When the regenerative amplifier is seeded with a 240 pJ, 3 ns optical pulse, output energy of 4.2 mJ with an overall energy gain more than 107 and output energy unstability better than 1% (root-mean-square) are obtained. When an extra post-pulse with amplitude of 0.75 times of the seed pulse is added for a 3 ns optical square seed pulse, the square-pulse distortion is decreased from 1.33 to 1.17.