Abstract: Cracks are easy to occur in low-carbon high-sulfur free cutting steel with Bi during high temperature deformation. The effect of B element on the hot deformation property of low-carbon high-sulfur free cutting steel with Bi was studied by high temperature forging and hot compression tests on Gleeble machine. The effect mechanism of boron on the hot deformation property of the test steel was analyzed by SEM, EDS, EBSD and other microscopic analysis methods. The results show that Bi tends to segregate at the grain boundary during hot deformation, which reduces the strength of the grain boundary and leads to the cracks. B can effectively improve the hot deformation property of low-carbon high-sulfur free cutting steel with Bi and avoid high temperature forging cracks. On the one hand, the beneficial effect of B is related to the softening effect of B, which makes the steel matrix easy to deform during hot deformation. On the other hand, it is related to the fact that B can promote the occurrence of dynamic recrystallization and further refines the grains.