As the multimedia social network develops rapidly, protocols protecting the security of realtime multimedia are becoming very significant. The ZRTP protocol, one of Real-time Transport Protocols (RTPs) for real-time multimedia applications, has many advantages compared with others. Though the calculation cost is relatively high, the ZRTP protocol provides perfect forward secrecy and authentication against man-in-the-middle attack. As so many formal verification tools were proposed recently, formal method has become one of the main methods of security protocol analysis. However, the formal verification tools are rarely used for the Real-time Transport Protocols. In this paper, we innovatively utilize the formal verification tool Scyther- Compromise and Tamarin to analyse the ZRTP protocol. According to the results, we find that the ZRTP protocol is insecure under the eCK model though it provides the perfect forward secrecy property. The adversary can impersonate the endpoint when the shared secrets are revealed. Besides, the experiments of this paper show that the formal method can perform perfectly and play an important role in protocol analysis.