We designed a novel surface gain slab laser combining the characteristics of slab lasers and disk lasers. A pumping system based on image relaying along the slow axis was used to obtain the uniform distribution of pumping light in the slab width direction. We achieved stable quasi-continuous laser output with the surface gain slab laser. The maximum output energy of 1064 nm laser pulse is 121.3 mJ when the pumping repetition rate is 300 Hz, the pulse width is 200 μs, and the pumping energy of a single pulse is 354 mJ. The corresponding optical-to-optical efficiency and the slope efficiency are 34.3% and 45%, respectively.