Hot charging of continuous casting slabs has the advantages of saving energy, reducing production cost and improving production efficiency. However, the surface of HSLA steel plate is prone to crack after hot charging rolling. In this paper, it was investigated that the microstructure, precipitation and crack of HSLA steel during reheating after hot charging at 700 ℃ and cold charging at room temperature, respectively. Then, the formation mechanism of hot charging cracks was analyzed by means of contrast experiment. The results show that carbonitrides of Nb, V and Ti tend to segregation at grain boundary and the ductility of grain boundary decreases greatly during hot charging, which is the primary cause of hot charging cracks formation.