The two factors which influence the low temperature performance of deformable mirrors (DMs) are the piezoelectric stroke of the actuators and the thermally induced surface deformation of the DM.A new theory was proposed to explain the thermally induced surface deformation of the DM:because the thermal strain between the actuators and the base leads to an additional moment according to the theory of plates,the base will be bent and the bowing base will result in an obvious surface deformation of the facesheet.The finite element method (FEM) was used to prove the theory.The results showed that the thermally induced surface deformation is mainly caused by the base deformation which is induced by the coefficient of thermal expansion (CTE) mismatching;when the facesheet has similar CTE with the actuators,the surface deformation of the DM would be smoother.Then an optimized DM design was adopted to reduce the surface deformation of the DMs at low temperature.The low temperature tests of two 61-element discrete PZT actuator sample deformable mirrors and the corresponding optimized DMs were conducted to verify the simulated results.The results showed that the optimized DMs perform well.