Information network security is very important for the Internet office. As a key component of information security, real-time network security risk assessment can effectively detect the security status of network resources. Therefore, in order to improve the security of office resources in the Internet environment, an online assessment method of network security risk based on prediction model is proposed. This method uses the expectation and maximization (EM) algorithm to improve the traditional continuous-time hidden Markov model in order to complete the risk assessment based on the predictive model. The simulation results show that the proposed method can effectively complete the network security online prediction. Compared with other methods, the proposed method can achieve high accuracy and real-time to meet the needs of a variety of information security under the Internet environment.