Current anonymous routing protocols do not provide anonymous mechanism for multicast in MANETs and have only had ad-hoc anonymity analysis. This paper proposes a new scheme called probabilistic multicast with universally composable anonymity. One-Time key pair is used to keep a route record in privacy during route discovery processes. Gossip-Scheme, secret DH path and Bloom Filter are adopted to realize anonymous source multicast during transmission of data packets. Finally, the protocol is analyzed based on the UC (universally composable) framework and its performance is evaluated by simulation. The analysis and simulation results show that the proposed scheme provides both anonymity and reliability for multicast in MANETs.