Abstract: Influences on the mode of positive-branch confocal unstable resonator are analyzed, which is caused by deformation of resonator mirrors. The coupling relations between deformation of resonator mirrors and laser modes are solved by using methods of time discrete, fast Fourier transform(FFT) and finite element analysis(FEA). With the increasing working time of laser discreteness, the results of calculation show that temperature increment and deformation of resonator mirrors become serious, which causes decline of laser modes and beam quality. The uniformity of phase and dispersing appearance are discussed. The correlative experiments show that laser output beam quality becomes worse when the laser works longer.