To obtain the relationship between impact toughness and fracture toughness, a curve relationship of the absorbed energy in the impact test, test temperature and ductile to brittle transition temperature was established. The expressions of fracture toughness of ship steel plates related to steel plate thicknesses and service temperatures were obtained. According to the PSN curve, the reliability was introduced, and the method to determine the corresponding fracture toughness of ship plate was realized under different steel plate thickness and service temperature in the arbitrary reliabilities condition, which can be a reference to make safe assessment of the cracked plate in ship structure.