At different temperatures the dynamic fracture toughness of a ship-building steel was tested and measured using the Hopkinson bar testing equipment. The rule of transition of dynamic fracture tough?ness versus temperature was first obtained，and the morphology of dynamic fracture at different tempera?tures was analyzed. Under a nigh loading rate, the characteristic of transition rule of dynamic fracture toughness versus temperature shows an upper and a lower shelf，and shows the sharp characteristics of duc?tile-brittle transition for the steel. With decreasing temperature the appearance of fracture of the steel is one of micro void coalescence, and then it has mixed cleavage and cleavage. The security assessment and the be?havior oi ductile-Brittle transition of the steel were obtained quantitatively.