We demonstrate a novel regenerative amplifier with a long ring cavity. Calculation on cavity parameters supports a large mode size. Based on the simulation with the cavity design software, we successfully established a 12 m ring regenerative amplifier. The amplified laser energy up to 20 mJ was obtained under 150 mJ green pump laser at a repetition rate of 10 Hz, corresponding to 13.3% conversion efficiency. The spectral bandwidth of the output pulse was 30 nm, which could support a sub 30 fs pulse duration. After seed injection, the ASE of the amplifier was effectively suppressed.