Thin disk lasers are becoming increasingly popular owing to their high peak power and pulse energy. Although high-power beams can be output by such lasers, the disk crystal is liable to deformation due to thermal effects, and the result is degraded beam quality. In this paper, a simple deformable mirror is designed and fabricated to compensate for the changes in the focal power of the thin disk crystal. The curvature radius of the deformable mirror can be changed between 2.050 m and +∞ by adjusting the pneumatic pressure in the air chamber. With the deformable mirror, the operating point of the thin disk laser can shift synchronously with the change in the focal power. The radius of the laser spot on the thin disk remains the same in the whole pump range. Consequently, the thin disk laser can operate in the fundamental mode, with the beam quality factor M² remaining no more than 1.2 under both the threshold and the maximum laser output conditions.