The heat generated in the cavity of the high power slab laser amplifier makes it less stable and induces a thermal-lens effect which adds a spherical phase distribution to the wave-front of the laser beam. The commonly employed interferometry or Hartmann–Shack sensor based measurements are unable to provide an accurate measurement of the thermal distortion of the gain medium. It is demonstrated that this problem can be solved using a scheme based on ptychographical iterative engine. The complex transmittance of the outgoing laser is obtained by the proposed scheme while the amplifier is set at its turned off and turned on state and the thermal distortion caused by the pumping light is calculated from the obtained transmittance.