In multi-agent systems, negotiations are important activities for solving agent conflicts and forming agent cooperation. Nowadays, many applications require multi-agent systems to work in open and dynamic environments. In such working environments, agents need to face various changeable factors in negotiations The dynamic features of open environments make the need of a flexible mechanism to handle agent negotiations that can involve various negotiation protocols and to allow agents to form dynamic negotiation relationships. In this article, we bring some advantages of Colored Petri Net techniques to capture dynamic and concurrent features of agent negotiations, and propose a Colored Petri Net based approach to model and handle agent negotiation activities. In this approach, agents can choose different protocols, which are represented in Colored Petri Net models, to operate negotiations. In addition, this approach supports flexible negotiations that involve different number of negotiation partners.