Abstract: The Leak Before Break（LBB） concept, as an important feature of the third generation nuclear power technology, is widely used in the high-energy piping design in nuclear industry. However, for some historical reasons, the application of LBB concept in China lagged behind compared with that in the developed countries. Up to now, there are no credible codes developed by China which can be applied to an actual project, and the relevant design mainly relies on foreign companies. Hence, the development of an approved LBB design code has great theoretical and practical importance for China. In this paper, the background of the fracture mechanics analysis, Crack Opening Displacement（COD） calculation and leak rate calculation in LBB design are briefly introduced firstly, and then the R＆D situation of the key code in NPIC is also presented, and some examples from the approved codes and published papers are used to verify the self-developed code. The calculation results show that the accuracy of the code is in well consistence with the examples. This code can be used in the practical engineering after refinement and validation.