Corner pumping method and gain module for high power slab laser are disclosed. In one embodiment, said method comprises directing a pump light from one or more pump light sources each consisting of a high power diode array and its coupling system into a laser slab through prior cut slab corners of said laser slab without restriction to the incident angle or the polarization state of the pump light, wherein said laser slab includes an undoped circumambient portion and one or more doped central portions; propagating said pump light within the laser slab by total internal reflection (TIR), wherein said pump light firstly pass said undoped circumambient portion, secondly pass said doped central portion, thirdly pass said undoped circumambient portion again, and fourthly take inner reflection at the surface of said undoped circumambient portion, and by repeating these steps, achieve multi-pass absorption; and substantially absorbing the pump light during propagating. In another embodiment, said gain module comprises a laser slab formed by solid state laser material, said laser slab including an undoped circumambient portion and one or more doped central portions and prior cut slab corners with corner faces; and one or more pump source providing a pump light; wherein said pump light is directed into said laser slab through said slab corners of said laser slab, propagated within said laser slab and substantially absorbed during propagation; and wherein said laser slab outputs an amplified laser beam.