In the field of artificial intelligence, problem understanding has always been one of the most important and difficult problems in the automatic solution of mathematical application problems. In this work, we systematically studied the problem of lacking commonsense knowledge in solving elementary mathematical problems, and constructed a commonsense knowledge base to assist computer in problem understanding. We collected the real and simulated questions of the classical probability for college entrance examination over the years as the research object. On the basis of in-depth analysis of topics, this study put forward the concept of commonsense knowledge definition, analyzed and classified its features, and stored commonsense knowledge with XML tags. On this basis, the commonsense knowledge base was applied in practice. The experimental results indicate that the commonsense knowledge base constructed in this study is very helpful for the automatic solution of classical probabilistic application problems.