Abstract: In irradiation surveillance tests of reactor pressure vessel (RPV) in nuclear power plants, easy-to-use straight through compact tensile fracture toughness C(T) specimens are used, and the strain is measured by mounting an extensometer on the front face of the specimen when testing. After the strain data is measured, the data is converted to load-line displacement by a specific inferring method. At present, there is no unified calculation method. In order to verify the validity of the different inferring methods, the deformation law of C(T) specimens was analyzed by finite element. The results show that the conversion method of front face deformation measurement and load-line displacement given in GB/T 21143-2014 was most consistent with the finite element results for the RPV material of nuclear power plant, and the accuracy of this inferring method was the highest.