A high power laser amplifier with high efficiency and compactness is designed. The thermal stress of symmetrical double cladding planar waveguide with Nd:YAG as its core is analyzed theoretically, and the maximum pump light intensity is also obtained. For different inner cladding thicknesses, the optimized pump sources and coupling system are achieved with the simulation of the TracePro software. In order to control the laser mode effectively, the 100 μm thickness of the core is set. The inner cladding and outer cladding are pure YAG and sapphire, respectively, with the waveguide of the dimension of 60 mm×10 mm×2 mm. The pump light from the laser diode array is coupled into the waveguide from two ends, two surfaces of the outer cladding are welded with two copper heat sinks for heat transfer. The seed light is coupled into the waveguide from one end and amplified for single pass. By theoretical calculation, the output of 1322 W can be obtained when the pump power is 3384 W and the seeder power into the waveguide is 0.1 W, with the optical-optical efficiency of 39%.