Network coding is an emergent promising technique that can be used to significantly enhance reliability in wireless multicast networks . In network coding schemes, information is partitioned into a number of blocks, which is the basic unit to perform network coding. In this paper, we will focus on the block delay model in lossy wireless multicast networks. Combining the 802.11 delay model and network coding muticast behavior, we give a theory model of average block delay. Simulations have been carried out to validate this model, and the numerical results are coincident to the theory model in 802.11 wireless networks.