According to the fatigue failure analysis of heat exchanger , on the basis of probability theory , combining the method of estimate of definite fatigue fracture mechanics , the formula to calculate the fatigue crack propagation life of heat exchanger' s components was deduced.The uncertainty and randomness of main assessment parameters were considered :initial crack size , practical working load , fracture toughness, mechanical intensity and coefficients.The random variables having some certain distributions were calculated with random sample by Mote -Carlo simulation method.A computer program was developed by using numerical integration method, and the fatigue life under given reliability and credit degree was solved .The result was compared with the fatigue life gained by theory computation method .The fatigue life in different initial crack sizes and different fracture toughness was compared , and an example was given.Analysis result shows that Mote -Carlo simulation method can reflect objectively the uncertainty of assessment parameters and overcome the shortcoming s of certainty assessment methods .So it plays a significant role in engineering application .