Cloud computing is growing in popularity among computing paradigms, and many corporate data centers are in the process of adopting a cloud computing architecture where resources are provisioned on a per-demand basis. The goal of a cloud infrastructure provider is to maximize its profit by minimizing the amount of violations of Quality-of-Service(QoS), and, at the same time, by lowering infrastructure costs. However, allocating virtual resources in cloud computing environment is very complex, and it's very difficult to manage virtual resources when the corporate data centers scale up. In this paper, we propose a dynamic constraint programming based virtual resources allocation model, which not only takes into account meeting user's QoS requirements, but also considers the cost of virtual cloud resources. We utilize constraint programming approach to solve the virtual resource provisioning and packing problem, and we validate our model through simulation experiments. We show, through simulation results, that our model is able to efficiently allocate and manage the virtual resources of the cloud platform. Compared with other traditional models, our model can significantly reduce QoS violations and achieve lower resource usage costs.