As the traditional workflow system in privilege management has the following weaknesses: low privilege management efficiency, overburdened for administrator, lack of trust authority etc. A secure workflow model based on PKI/PMI is proposed after studying security requirements of the workflow systems in-depth. This model can achieve static and dynamic authorization after verifying user's ID through PKC and validating user's privilege information by using AC in workflow system. Practice shows that this system can meet the security requirements of WfMS. Moreover, it can not only improve system security, but also ensures integrity, confidentiality, availability and non-repudiation of the data in the system.