At present, the social media has been people's top priority for social apps using. However, because of their spreading mechanism and characteristics, criminals utilize social media as the seedbeds of their illegal activities which greatly threatened people's security. Primarily, there are four aspects: first, terrorist organizations and individuals use social media as a tool to promote terrorist activities; second, criminals use social media to commit crimes such as sex gambling and fraud; third, social media has become a bridge to crime; fourth, social media plays an important role in guiding the online public opinion.