In order to calculate the three-dimensional transient temperature field of the continuous moving laser molten pool, a numeric model is established basing on ANSYS software. In the established model, the influences of the surface temperature of material on laser absorptivity and phase change of material on temperature field of laser molten pool are taken into consideration. The analysis on the time-depended temperature field of laser molten pool is also conducted. During laser processing, the heating and cooling process of laser molten pool can be acquired by the numeric model. The calculated results show that when lasers scanning on the top surface of 45# steel base plate from one point to the another point along straight line, due to thermal conductivity, the temperature of laser molten pool increases with time increasing. The calculated result also indicates that the position of the highest temperature point on the laser molten pool surface is slightly lagged the position of the center point of the laser beam. Under the same laser process parameter conditions, the calculated result agrees with the experimental result, this indicates that the established numeric model is correct and reliable.