An attempt is made to give a physical picture of the dynamic thermal deformation in thin films under transient laser pulse heating, which has been observed by one of the authors previously. To simplify the real problem, the transient bulk heating model is used, then using Lord-Shulman theory, the dynamic thermal strain as a function of time and relaxation time is derived. For a thin film, the theoretical calculation shows the same result as the experiment. The present analysis is a simple method which is particularly suitable for systems with a random metallic structure and thin samples.