5G refers to the next generation of telecommunication techniques facing the year of 2020. It aims to offer 100 times faster data transmission rate than current Long Term Evolution (LTE) networks. To achieve this goal, recent research proposed to apply the interoperability among heterogeneous wireless networks to improve the data rate. However, such an approach to detecting free bandwidths and efficiently managing forwarding packets is still in its infancy. In this article, we propose a Multiple Radio Access Networks (Multi-RANs) parallel accessing architecture (named SDN-5G) by importing Software Defined Networking (SDN). It can optimally utilize the multiple RAN capabilities of mobile devices to improve the data transmission rate. Moreover, we analyze the security of the proposed architecture, list possible security threats, and design a number of security schemes to protect the proposed SDN-5G networks. We also give the security proofs of the each scheme with Burrows-Abadi-Needham (BAN) logic, and verify the security of the mutual authentication and key distributions of the proposed SDN-5G architecture.