In this paper, a minimum toughness assessment method of exempting postweld heat treatment (PWHT) based on the master curve method is proposed to replace the current method of determining whether to perform PWHT by material thickness. Reference temperature to prevent fracture (T0-required) for structure to be exempt from PWHT at minimum design metal temperature (MDMT) was obtained by using the master curve method under presumed stresses and flaw size. A series of exemption curves were generated for the steels used in pressure vessels with yield strength of 300∼550 MPa, and the method was corrected for low-stress conditions. Then the feasibility of this method was verified by an engineering example. A SA-738Gr.B welded joint in the example was taken as the research object, the experiment showed that the weld metal was the worst part of the fracture toughness of the joint. The results of research objects which were exempted from PWHT by the proposed method are consistent with ASME Code Case N-841, which proves that this method is feasible in assessing the exemption from PWHT.