Abstract: A comprehensive analysis method has been developed to study the transient response of a thermal-viscoplastic thin plate subjected to constant loading and rapid heat input. Thermal deformation and failure of 4340 steel bar is analytically investigated under various stress levels and laser power densities. The present model can be used to determine parameters related with critical initiation of rupture, such as deposited laser energy, loading, and duration of heating and so on. The threshold time of heating, threshold temperature of material and threshold energy for laser thermal heating coupled with the tensile stresses causing the structural failure is calculated. The calculated results are consistent with the experiments. The results show that the deformation and failure mode of structure is mainly dependent on the heating duration, and there have three modes: rapid rupture, no rupture and delayed rupture. In general, the higher the loading is, the shorter the rupture time is, and the less the temperature of the material. The laser power density has no evident influence on the consumed energy of the ruptured material.