Abstract: A 3D model for fluid flow and heat transfer process has been developed to predict the transient distribution of temperature and velocity of a thick solid irradiated by a moving laser.The governing equations are solved by means of a finite difference on the self adaptive grids.The results show that the unsteady state process of fluid flow and heat transfer in laser molten pool is a preheating process as the highest surface temperature,depth and radius of the molten pool increase with the scanning time.According to the sequence of time,the unsteady state process can be divided into three stages:the initial stage,the quasi state stage and the rapidly rising temperature stage.At quasi state stage,there is a little increase in the dimension and the highest surface temperature of the molten pool,the staying time is much longer than those of the other stages,which show that 3D quasi state is a good approximation to 3D unsteady state model.The calculated results are consistent with those obtained by Wang.