Aiming at improving the beam quality of thin-wall tube laser, a novel method based on the right-angle cone deformable mirror is proposed. In the method, a reflector with inner right-angle conical surface is used, and the annular laser beam successively passes through the opposite sides of the tube, compensating for the off-axis aberrations of the annular laser beam. Next, the residual aberrations are corrected by the deformation of the right-angle cone mirror to further improve the beam quality. The physical model of the right-angle cone deformable mirror is built up by using the finite element analysis method, followed by optimizing the structural parameters of the right-angle cone deformable mirror. The preliminarily optimized right-angle cone deformable mirror drived by 48 actuators with a radius of 1.5 mm for each actuator and an interval of 11 mm between actuators is then utilized to correct the beam quality of the thin-wall tube laser. Results indicate that the output beam quality of the thin-wall tube laser degrades rapidly with the increasing of the tube’s concentricity error, parallelism error, taper error and source’s parallelism error. Fortunately, the beam quality is significantly improved by using the right-angle cone deformable mirror and the β factor greatly decreases. In addition, the performance of the non-ideal right-angle cone deformable mirror with a 20-μrad taper error and a 10-mrad collimation error is compared with that of the ideal mirror, and the results show that the β factor is controlled within 1.14 after having been corrected by the non-ideal right-angle cone deformable mirror. Therefore, the simulation results theoretically prove that the novel method can effectively eliminate the typical aberrations caused by the errors from fabrication and alignment and correct the wavefront distortion of the large-aperture thin-wall tube laser, thus significantly improving the beam quality.