Based on thermo-elastic theory, an analytic model is developed for determining the laser fluence required to damage a series of thin glass disks which serve as transmissive optical components. In particular, the dependence of threshold fluence on laser beam diameter and mechanical boundary conditions of samples is investigated. Study shows that beam diameter effect in material response results from the energy loss of the irradiated area. As for structural response, different mechanical boundaries result in different relationships of LIDT(laser induced damage threshold) with radius of laser beam. It is also found that mechanical boundaries have great influence on LIDT even if beam diameter is fixed, and optimal boundary under which tensile stress is eliminated is available theoretically when particular compressive boundary is applied.