Abstract: In this paper,by weld themo-simulation test with different peak temperatures and t_8/5,the variation of HAZ properties of 10Ni5CrMoV steel was studied.The results obtained from the test showed that when the peak temperature of single thermal cycle increased,the HAZ impact toughness decreased, when t_8/5≤40s,the impact toughness increased with t_8/5 raising,but when t_8/5 40s, the HAZ impact to ughness evidently decreased with t_8/5 raising.The variation of temperature in repeately thermal cycle had no remarkable effect on the impact toughness.But when the peak temperature of the thermal cycle was 1350℃ firstly and 950℃ secondly,thehistogenetic phenomenon appeared in the HAZ.It's features and producing condition were explained in this paper.