The Space-Air-Ground Integrated Net-work (SAGIN) realizes the integration of space,air,and ground networks,obtaining the global commu-nication coverage.Software-Defined Networking(SDN) architecture in SAGIN has become a promis-ing solution to guarantee the Quality of Service (QoS).However,the current routing algorithms mainly focus on the QoS of the service,rarely considering the secu-rity requirement of flow.To realize the secure trans-mission of flows in SAGIN,we propose an intelli-gent flow forwarding scheme with endogenous secu-rity based on Mimic Defense (ESMD-Flow).In this scheme,SDN controller will evaluate the reliability of nodes and links,isolate malicious nodes based on the reliability evaluation value,and adapt multipath rout-ing strategy to ensure that flows are always forwarded along the most reliable multiple paths.In addition,in order to meet the security requirement of flows,we in-troduce the programming data plane to design a multi-protocol forwarding strategy for realizing the multi-protocol dynamic forwarding of flows.ESMD-Flow can reduce the network attack surface and improve the secure transmission capability of flows by im-plementing multipath routing and multi-protocol hy-brid forwarding mechanism.The extensive simula-tions demonstrate that ESMD-Flow can significantly improve the average path reliability for routing and in-crease the difficulty of network eavesdropping while improving the network throughput and reducing the average packet delay.