Abstract: The overall performance and service life of engineering equipment can be greatly improved for the heavy use of high-strength steel, but it does not change the fact that the weak performance area after welding is still the heat affected zone(HAZ). By the proper rare earth treatment technology, not only the rare earth inclusions with beneficial morphology, size and distribution can be formed in the steel, but also the growth of original austenite grain can be restrained during the welding process. It is actually for these reasons that the welding performance of the steel are boosted.In this study, a kind of 0.18% C high-strength steel with rare earth is produced. The thermal cycling processes under 4 kinds of heat input are simulated by the Gleeble-3500 thermal simulator. The microstructure transformation of HAZ is observed by the optical microscope. The impact absorption energy of HAZ is measured by the testing machine. The changes of original austenite grain size under different cooling rates are measured. The results show that when the welding heat input is 25 kJ/cm, the HAZ microstructure is mainly martensite with fine grain size, and then the impact toughness and hardness are the highest level. When the heat input is higher than 50 kJ/cm, the upper bainite and granular bainite are formed, and the grains grow up gradually, resulting in the decrease of both toughness and hardness. The C content of the test steel is 0.18%. During the thermal cycle the coarse martensite structure appears, forming the harden structure without the acicular ferrite structure.