On the base of analyzing the thermal lensing effect caused by the temperature-gradient in direction of unpolished-face, a new type construction consisted of absorbing glass, insulative glass and heat sink has been designed for adiabatic structure of slab, and the experiment of thermal focal length of slab has been done. The result proves that the thermal lensing effect of slab has been improved due to adopt adiabatic construction. The focal length of 1.93m in direction of unpolished-face has been measured from the slab with adiabatic construction at 500Hz, comparing with 0.78m measured from the slab without adiabatic construction.