Abstract: Cooling transformation behaviors of Q500q bridge steel by two cooling modes were studied with Gleeble-2000 thermal simulation machine. One cooling mode method (constant cooling) was cooling from 900℃ to 300℃ at constant cooling rates (1, 4, 8, 16, 32℃·s^-1) and the other cooling mode (subsection cooling) was cooling from 900℃ to 650℃ at constant cooling rates (ditto) and then to 300℃ at halving cooling rates. The results show that under constant cooling conditions, the steel had a structure of polygonal ferrite+pearlite at the cooling rates of 1-4℃·s^-1, a structure of bainite at the cooling rates of 8-16℃·s^-1, and a structure of martensite at the cooling rates of 32℃·s^-1. Under subsection cooling conditions, the ferrite and pearlite transformation occurred at the cooling rates in the intermediate temperature range (650-300℃) of 0.5-4℃·s^-1, and the amount of polygonal ferrite was relatively large; the bainite transformation occurred at the cooling rates in the intermediate temperature range (650-300℃) of 8-16℃·s^-1. Compared with those by constant cooling method, the bainite transformation end temperature increased, the transformation region was reduced, and the grain refinement degree of bainite decreased.