A mode program is established to investigate the thermal distortion of the Nd : glass rod amplifier from its deposited energy distribution. This program can be used to calculate the temperature profile, stresses distribution, refractive index distribution, depolarization and phase aberration during the pumping and the cooling down processes. The influence of the pumping on laser wavefront has been measured with a Hartmann wavefront sensor. Meanwhile, the transient temperature profile has also been measured by the Hartmann wavefront sensor. The experiment is in good agreement with the calculation.