Due to system resources and realtime constraints，the security design methods for general computing systems are not fit for the realtime embedded systems,so,it is necessary to study the special design method. To obtain the optimal security of embedded systems under realtime constraints,a complete set of security design methods which include the securityrelated task graph model and the security evaluation model are proposed in this paper. Based on these,an ILP based optimal security strategy generation method named ILPOS is also proposed. This method can select security algorithms and detect the realtime schedulability simultaneously. That is more efficient than the phased coupled optimization methods,therefore,it can make full use of the system time for security extension. The experimental results show that ILPOS can improve the system security efficiently under various time constraints compared with the traditional heuristic algorithms.