The laser cladding process on tooth surface of a helical gear shaft which is the rotor in lubricant pump of steam turbine is investigated. Temperature field is numerically simulated with finite element analysis software-ANSYS. Calculation results show that the heat caused by laser energy is concentrated inside the tooth mostly, and the distribution of isothermal lines is very dense at the binding site of clad coating and tooth root. Large temperature gradient would be generated inside the tooth. In addition, an effect of preheating in the adjacent tooth would also be brought up by laser energy. It is also indicated that the isothermal lines are penetrated into the tooth tip, and the problems of excessive melted down and collapsing of tooth tip would be easily brought about if the tooth side and tooth tip are cladded at the same time. So the tooth side and tooth tip should be cladded step by step. Continuous and complete clad coating on tooth surface is produced in experiment by this technology.