Abstract: Mobile Ad hoc NETworks (MANET) has inherent characteristic of dynamic topology, distributed collaboration, thus on-demand routing protocols employs multicast mechanism to improve transmission efficiency. However multicast is apt to cause network congestion which makes congestion condition in MANET critical. On the basis of the stochastic characteristics of MANET node and 802.11 DCF basic access mechanisms, the paper proposes an infinite state quasi-birth-and-death model to accurately describe the packet generating, queuing, sending process of individual node. Stationary analysis is performed with matrix-geometric method and the set of equations for packet arrival rate threshold is obtained. The effectiveness of this model is demonstrated by the simulations in OPNET. The contribution of this paper is to provide a mathematical tool for congestion control research.