Abstract: Fatigue fracture often happens in welded joint with high strength in engineering practice.The reliability of weld anti fatigue in safe shonld be estimated.According to fatigue fracture theory of metal material and drift principle in reliability technology, the method to calcalate the reliability under the failure rate with a certain cycle numbers or under the cycle numbers without failure was discussed by using crack-length before weld fatigue fracture as output parameter.A praitical example was used to design the reliability of anti fatigue failure strength of weld under the condition of the given cycle numbers and reliability.