For the purpose of exploring the welding suitable heat input and formulating the actual welding procedures, the effect of welding heat input on the phase transformation, microstructure and toughness of coarse heat affected zone (CGHAZ) of NbTi microalloyed steel had been studied by using FormastorF Ⅱ full automatic transformation measuring apparatus and Gleeble3800 thermal simulator. The results showed that the microstructure of CGHAZ was granular bainite and lath bainite, when the cooling rate was larger than 6 ℃/s. With the decrease of cooling rate, the proportion of lath bainite decreased and the proportion of granular bainite increased. At the cooling rate of 6 ℃/s, the grain boundary ferrite formed and the proportion increased as the cooling rate furtherly decreased. When the cooling rate was no more than 0.6 ℃/s, the microstructure was composed by ferrite and pearlite. With the increase of t8/5, the MA constitution increased firstly and then decreased. At the t8/5 of 178 s, the content of MA reached a maximum of 5.1%. When the welding heat input range was 144178 s, the content of MA component was the main factor controlling the toughness of NbTi steel in simulating CGHAZ zone. As the t8/5 was equal to 256 s, the content of MA decreased, and the coarsening grain boundary ferrite was the main factor influencing the CGHAZ toughness.