From the aspect of system theory, this paper studies the mobile office model, and defines the relevant concepts of mobile office system clearly, such as framework, basic elements and their roles as well as their interrelationships. In this paper, the risk factors and characteristics existing in mobile office activities are analyzed, the risk assessment method from different angles and starting points is explored, the selection principle and method of evaluation indexes is discussed, and a fuzzy comprehensive evaluation method is put forward to verify the safety and reliability of our mobile office system.