Intrusion detection technology is a kind of network security technology that can protect system from attacks. Based on the definition of system call risk coefficient, the paper brought out a system risk coefficient based dynamic intrusion detection model. Using the model, the drawbacks of traditional intrusion detection method based on system call was solved, which speeds up detection process and decreased false rate and error rate. It can also effectively identify error operations or users. The experiment result also proves the effectiveness and efficiency of the method.