Abstract. Vulnerabilities usually represents the risk level of software, therefore, it is of high value to predict vulnerabilities so as to evaluate the security level of software. Current researches mainly focus on predicting the number of vulnerabilities or the occurrence time of vulnerabilities, however, to our best knowledge, there are no other researches focusing on the prediction of vulnerabilities' severity, which we think is an important aspect reflecting vulnerabilities and software security. To compensate for this deficiency, we propose a novel method based on grey system theory to predict the severity of vulnerabilities. The experiment is carried on the real data collected from CVE and proves the feasibility of our predicting method.