Software defined networking (SDN), which separates data forwarding from control, is a complete overthrow of traditional network architecture, introducing new opportunities and challenges for all aspects of network research. With the traditional network research methods encountering bottlenecks in SDN, deep learning based methods have been introduced into the research of SDN, resulting in plenty of achievements in real-time intelligent network management and control, which promotes the further development of SDN research. This study investigates the promoting factors of introducing deep learning into SDN, such as deep learning development platform, training datasets and intelligent SDN architectures; introduces the deep learning applications in SDN research fields such as intelligent routing, intrusion detection, traffic perception, and other applications systematically, and analyzes the features and shortcomings of those deep learning applications in detail. Finally, the future research direction and trend of SDN are prospected.