It is a conventional method to regard laser iradiated object as a semi infinite medium with invariable thermal physical property parameters for approximate analyses of thermal acting of laser heat treatment. So far, 2αt and α/t have been taken as the thermal diffusion distance and the thermal diffusion velocity respectively, at the time t after heat fluctuation in materials with a thermal diffusion coefficient α, and as a result, the feasibility of the approximately estimating method about semi infinite medium is judged. This paper indicates that, due to quantitative investigation of heat energy diffusion, the thermal diffusion distance of 3αt and the thermal diffusion velocity of 1.5α/t can more accurately describe the physical process of heat conduction. Finally, an applied example of approximately estimating the temperature field of laser heat treatment is presented. It is a conventional method to regard laser iradiated object as a semi-infinite medium with invariable thermal physical property parameters for approximate analyzes of thermal acting of laser heat treatment. So far, 2αt and α / t have been taken as the thermal diffusion distance and the thermal diffusion velocity respectively, at the time  t after heat fluctuation in materials with a thermal diffusion coefficient α,  and as a result, the feasibility of the approximate estimating method about semi infinite medium is judged. This paper indicates that due to quantitative investigation of heat energy diffusion, the thermal diffusion distance of 3αt and the thermal diffusion velocity of  1.5α / t more more describe the physical process of heat conduction. Finally, an applied example of approximately estimating the temperature field of laser heat treatment is presented.