The Bayesianism, which arose in the 1930s on the basis of the Bayes theorem and revived in the 1990s, has become the important research program and the hotspot of scientific methodology. In the field of scientific reasoning, the Bayesianism has the following advantages, i.e. formalizing the important intuitions of the traditional scientific methodology, avoiding the difficulties in the traditional scientific methodology, and resolving the “mysteries” of the traditional scientific methodology. But it also has some dilemmas, such as old evidence, logical omniscience, simplicity, countable additivity, and so on. Currently, the researches on the Bayesianism study has m mainly anifested in two aspects, that is the Bayesian inference in cognitive science, and the Bayesian network in artificial intelligence.