Continuous faceplate discrete actuator deformable mirror (CFDA-DM) is the most important device in adaptive optics. In high power laser systems, this kind of deformable mirror (DM) is apt to produce thermal deformation, which can seriously affect the compensation ability of DM and the output beam quality. Experimental platform is set up to study DM thermal deformation. In the experiment, when DM is continuously irradiated by 2.5 kW laser for 130 s, the peak-to-valley value (dPV) of wavefront increases from 0.0806λ (before irradiation) to 0.5423λ, the higher order aberration of faceplate is 0.1220λ, and the low order aberration is mainly defocus and astigmatism. After dPV of a zero degree astigmatism is compensated from 0.4853λ to 0.0707λ, DM is irradiated by 2.5 kW laser for 180 s again. In consequence, the dPV of wavefront increases from 0.0707λ to 0.7548λ, and the higher order aberration of wavefront is 0.2487λ. The above results show that the thermal deformation does not only lead to lower order aberration, but also produce higher order aberration, which cannot be compensated by DM itself. The higher order aberration is the key factor that influences the output beam quality of high power laser systems.