To obtain high energy, high efficiency Ndglass preamplifier, a Ndglass regenerative amplifier system is designed. By adjusting the single pass gain and carefully optimizing the cavity mode, the small-scale self-focusing effect in the gain medium is effectively controlled. Maximum pulse energy of 21 mJ, pulse width of 2.65 ns is obtained at the repetition frequency of 1 Hz, corresponding to a high optical-optical conversion efficiency of 5% and amplification ratio of 108. The root-mean-square (RMS) of pulse to pulse energy stability is less than 2% during 2 h continuous operation. The laser has a good beam quality of M2=1.5. The spectrum is measured to be at center wavelength of 1052.92 nm.