Multiple collinear surface cracks distributed that are randomly along a weld toe have a strong influence on the fatigue crack propagation life of welded joints. This issue is investigated using statistical approaches based on series of systematic experiments, in which initial crack numbers, their locations and crack sizes, i.e. depth and length, are taken into account. The number of initial cracks follow a normal distribution, and the probability of initial crack depths and lengths can be accurately described by the Weibull distribution. These characteristics are used to calculate the fatigue crack propagation life, in which the mechanisms of the mutual interaction and the coalescence of multiple cracks are considered as well as the Mk-factors. The automatic calculation of fatigue crack propagation life is achieved by the application of NESUSS, where parameters such as the number, location and size of cracks are all treated as random variables. The random variables are dealt through a Monte-Carlo simulation with sampling random numbers of 2,000. The results of the simulation provide the statistical characteristics of the fatigue crack propagation life for welded joints as a function of the number of initial cracks. The sum of the simulation results and the fatigue crack initiation life referred from a previous paper is in good agreement with the experimental results.