A Q-switched Nd:YAG/Cr4+:YAG laser is used to seed a one-stage amplifier featuring backward pumped Yb-doped double-cladding fiber as the power amplifier. Average power of 5.5 W with repetition rate of 9 kHz is achieved, corresponding to pulse duration of 575 ps and peak power of 1 MW. Based on the fiber amplifier at 2.7 W, 1.3 W green laser is obtained by single-pass frequency doubling using non-critical phase matching LBO crystal at 150.5 ℃, and its peak power reaches 250 kW, with conversion efficiency of 48%.