We　present　a　novel　structure　of　planar　waveguide　for　Yb:YAG　laser　amplifiers　as　symmetrical　corner-pumped　planar　waveguide,　allowing　large　absorption　length.　We　optimize　the　parameters　of　the　waveguide　using　an　amplification　model.　Performance　including　thermal　effects　of　10-kW　symmetrical　corner-pumped　planar　waveguide　laser　amplifier　is　simulated,　demonstrating　that　pump　absorption　efficiency　and　uniformity　are　much　better　than　end-pumped　ones.　Moreover,　a　good　beam　quality　causing　by　less　thermal　distortion　can　be　expected.　The　power　is　scaled　from　200W　to　7000W　with　the　pump　absorption　efficiency　of　98.2%,　leading　to　an　optical-to-optical　efficiency　of　68%.