Abstract: The thought is proposed that reliability safety assessment based on R6 regulation with failure probability index should be process, in accordance with the uncertainty of the parameters of r6 safety assessment and the organ relation of material and flaws and stress. The key technique probabilitizing R6 regulation is resolved by using the 2 level method to calculate failure probability and proposing so called three – phase method which convergent quickly and has highly precision to obtain the minimum value with complex goal function. Finally, the reliability safety assessment method to pressure pipe is recommend.

Key words: Failure probability 2 level method, Safety assessment, Safety grade