Contract net protocol is widely used in multi-agent systems because of its highextensibility and flexibility in a dynamic environment. However, many shortages exist while applying the traditional contract net protocol in multi-agent systems, such as rapidly increasing of the negotiation cost along with the system's dimension, no guarantee of the negotiation quality and difficulties in adapting the dynamic environment, etc. A multi-agent negotiation model based on the acquaintance coalition and the extended contract net protocol are presented to improve the efficiency of negotiation. The structure of the multi-agent negotiation model is given to support the extended contract net protocol. The acquaintance coalition policy, the trust degree parameter and the adjustment rule are introduced and used to construct the extended contract net protocol. Finally, through an example and analysis of a missile defense system which uses the model, improvement of the negotiation efficiency is proven.