We　present　a　novel　structure　of　planar　waveguide　for　Yb:YAG　laser　amplifiers　allowing　large　absorption　length.　The　structure　parameters　of　the　planar　waveguide　were　optimized　using　an　3D　amplification　model.　The　performance　of　the　optimized　planar　waveguide　laser　amplifier　was　simulated,　and　a　comparison　between　the　cases　with　and　without　considering　pump　saturation　was　carried　out.　The　simulated　results　show　that　a　high　pump　absorption　efficiency　and　optical-to-optical　efficiency　can　be　expected　in　addition　to　a　good　absorption　uniformity.　The　seeder　power　is　scaled　from　200W　to　7000W,　the　corresponding　pump　absorption　and　optical-to-optical　efficiencies　are　98.6%　and　68%,　respectively.　The　thermal　stress　of　the　designed　planar　waveguide　was　analyzed　theoretically,　showing　that　the　pump　power　of　10kW　is　available　according　to　the　stress　fracture　limit.　©　2022　SPIE