Abstract: Non-destructive testing (NDT) is an important mean to ensure the welding quality of nuclear island machinery. The domestic welding acceptance standards of nuclear power plants are from foreign classic codes including ASME and RCC-M. However, basis principles of formulating the acceptance criteria are unclear. Based on systematical comparison of NDT acceptance criteria in major standards, it is found that the NDT acceptance standards in ASME and RCC-M are similar since RCC-M referred the system and principles of ASME, although there are several variations in detail. But the two codes both differ greatly from other welding acceptance criteria such as PNAE G-7-010-89 and ISO 5817. These acceptance criteria are mainly based on engineering experience at the time. This leads to inconsistencies in the definition of so-called “weld class systems” which were primarily developed based on concepts related to good workmanship but have little or no relation to the actual performance of the welded structure. The difference in the design standard system also affects the formulation of the NDT acceptance criteria. With the development of fracture mechanics theory and wide application of computer technology, flaw assessment methods based on fracture mechanics were developed. In the in-service inspection process, flaw assessment methods were used to replace the traditional welding quality assessment standards, while the innovation during the manufacturing is not tried in nuclear industry yet. In order to apply the flaw assessment methods to determine the welding acceptance criteria for non-destructive testing of high temperature reactors, following principles should be considered. First, there is no need to formulate different acceptance standards according to non-destructive testing methods, and only the nature of the defect itself, such as type, location, size, and quantity, should be distinguished. Second, considering the consistency of standard in use, the acceptance criteria should also be consistent with the welding evaluation method of design code. Finally, from the perspectives of fracture mechanics and fatigue damage, the influence of welding defects on the mechanical properties could be analyzed and summarized to set up the relationship between the volumetric defects and service life. Combined with the characteristics of whether the equipment can be replaced and whether it is easy to monitor in service, one can determine the acceptance limits with different margins. The technical principles of the method are instructively important for determining an advanced, reasonable, and economic welding NDT acceptance criteria of advanced reactors.