Abstract: Carbon extraction replica analysis was employed to measure the quantities and size distributions of particles in Ti microalboyed steel and its simuated CGHAZ to reveal the response behavior of second phase particle to the welding thermal cycle. And the microstmcture and impact toughness of CGHAZs in Ti microalloyed steel and a kind of HSLA steel with similar chemical content and manufacture process were investigated with OM, TEM and series charpy impact test. The results show that a large amount of small-size TiN particles exist in the Ti microalboyed steel. During the weld thermal cycling, TiN particles prohibit the original austenite grain from coarsening, prevent the formation of coarse upper bainite, accelerate the formation of acicular ferrite, and promote the decomposition of M-A constituent into ferrite and cementite, thus improve the toughness of CGHAZ in HSI.A steel remarkably. With the t_8/5 increasing from 16s to 120 s, the toughness of CGHAZ in Ti microalloyed steel increased because the secondary microstructure of CGHAZ changes from upper bainite to acicular ferrite and the austenite grain size coarsening is not serious.