Ultra-thin slab with the dimension of 1*10*60 (mm) is demonstrated to acquire high-power laser output with high beam quality. The output average power of 70 W with diffraction limited beam quality is obtained, and the total optical-optical efficiency is about 32%. It can produce more than 100 W output laser when it operates with multi-mode. Cr4+:YAG is adopted as the saturable absorber, the pulse repetition rate is higher than 10 kHz, and the pulse duration is less than 10 ns. An asymmetrical composite crystal is also studied, the new crystal consists of two layers: one is undoped YAG, the other is Nd-doped YAG layer. It is shown that thermal effects of the new asymmetrical crystal are obviously reduced.