In the open network environment, the application systems of mobile agent, such as the process of data and codes of mobile agent transmission, server system resources, mobile agent runtime platforms and mobile agent itself, have to face many hidden security troubles and dangers. We analyzes the security problems resulting from mobile agent and then summarize the present solving methods of the following two aspects: (1) protect server resources; (2) protect mobile agent. At the end of the article, it gives an elementary security model of mobile agent system.