Due to the broadcast nature of wireless radio propagation channels, 5G wireless networks face serious security threats. Security mechanisms that leverage physical layer characteristics have been considered as potential complements to enhance 5G wireless network security. This article proposes physical layer security enhancements to defend against security attacks from the three most significant aspects: wireless secure communication, physical layer assisted authentication and secret key distribution. By integrating physical layer security with novel techniques and application scenarios in 5G communication, we elaborate several promising strategies, including massive MIMO beamforming with security code communication, self-adaptive mobile and cooperative secure communication with dynamic channel prediction, physical layer assisted authentication, malicious node detection and key generation of massive MIMO millimeter wave system. Research opportunities and future challenges are further discussed.