Abstract: The numerical elastic-plastic analyses were processed by using FEA to three-point bend sam-ples in the ductile-brittle transition range.The samples are made of 22NiMoCr3-7 pressure vessel steel with single edge crack.Stress fields and constraint effects of the standard crack sample and the sallow crack sam-ple was assessed by using J-A₂ fracture theory.And the constraint parameters A₂ relating to the master curve reference temperatures T₀ were obtained.Based on J-A₂ approach and RKR fracture model,a formula to fore-cast fracture toughness was given.When it was used to forecast the fracture failure curves at different tem-peratures,the forecast results agree well with the experimental data.