According to the characteristics of the existence of keyhole phenomena on TC4 titanium alloy during laser welding, a precise calculation model fitting to the keyhole size in micron dimension has been developed. Using the Gaussian rotating curved body heat source fitting for the keyhole profile and combining the material property of TC4 titanium alloy, the distribution characteristics of keyhole profile and transient welding temperature varying with the laser parameters have obtained. The greatest influences of laser welding parameters on keyhole profile of TC4 titanium alloy have been found out by analyzing the specificity size and temperature distribution in front and back edge of the keyhole and in radial and depth direction of the keyhole. The optimal range of laser welding parameters to obtain the best keyhole profile and temperature distribution of TC4 titanium alloy during laser welding has been found out by analyzing the influence of every welding parameter.