A design of unstable ring resonator with 90°(UR90) beam rotation employed in ceramic YAG disk laser is presented .The system of beam rotation is built up by a prism and two thin ceramic YAG disks which are placed vertically. Fast Fourier transform method(FFT) is used to simulate the mode of UR90 beam rotation. The effect of inhomogeneous gain on laser mode of an UR90 beam rotation is also analyzed. For two laser-diode side-pumped disk laser, the difference between the mode of UR90 and normal unstable resonator is compared. It provides evidences that UR90 beam rotation adopted by solid-state laser is feasible.