Abstract: In this paper, a new type of heat-resistant steel, P92 was used as the research object, and the fracture toughness at 630 °C high temperature was studied by using different kinds size of side-grooved and plain-sided of compact tension(CT) specimens. The resistance curves and the corresponding fracture toughness J_Q of the specimens under different forms were obtained. Through the analysis of fracture morphology, the fracture toughness of P92 steel at high temperature is good, which is a typical ductile fracture mechanism. Based on the three-dimensional finite element calculation, the influence of the side-groove on the constraint of the specimen was characterized by constraint parameter T_z and Q, through which the effect of the constraint on the experimental results was discussed. Research shows that, side-groove can significantly increase the constraint level of the specimen, and the smaller the size of the specimen, the more obvious the effect of the side-groove, so the smaller the size of the specimen, the more obvious the difference between the J resistance curve; with the increase of load, plain-sided specimen's constraint change is more obvious, so side-groove structure will lead to different resistance curve; the change of the size or structure of specimens has a great influence on the resistance curve of the ductile material, but the effect on the fracture toughness value is small. After the side-grooved, the crack propagation is more uniform and the fracture toughness test process can be optimized.