Abstract: In the flue gas energy recovery system of catalytic cracking unit,the opening of slip valve is a key parameter.The bigger its value,the safer the system, however,the higher the energy consumed.In this paper,the risk decision theory is used to decide the safe opening of slip valve.Based on the risk probability analysis by Delphi inquiry,which is further corrected by Bayes’ theory, the opening value of the valve is determined by means of the decision tree method.It was used satisfactorily at Jinzhou Petro-Chemical Co.