Abstract: Taking the Φ339.7 mm×13.06 mm TP140V casing as a study object，the crack tip opening displacement of the casing crack specimen is measured by the three point bending test.Analyzed are the crack propagation resistance curve and its characteristic values，and so determined are the theoretical conditions for crack propagation of the casing.It is found that the CTOD values of both the casing body and the coupling measured in the test are higher than the theoretical values，which indicates that the fracture toughness of the casing can meet the specific field working condition.