In this paper we introduced a method to exchange the private key by adopting steganography in RTCP packet for the covert communication based on VoIP. We presented a model of covert communication based on VoIP, and pointed out the significance of key exchanging to the covert communication. Then, we turned to suggest a steganography algorithm to hide the private key in RTCP packet, and analyzed the performance of the algorithm. To overcome the packet loss, a scheme of key pairs was suggested in case the previous key was lost in transmission. Finally, we analyzed the performance of this scheme, and find that the steganographic key exchanging could be finished in about 1 second even when the packet loss rate is as high as 5% in VoIP using G.711 codec.