Fog computing is an emerging par-adigm of cloud computing which to meet the growing computation demand of mobile appli-cation. It can help mobile devices to overcome resource constraints by offloading the compu-tationally intensive tasks to cloud servers. The challenge of the cloud is to minimize the time of data transfer and task execution to the user, whose location changes owing to mobility, and the energy consumption for the mobile device. To provide satisfactory computation performance is particularly challenging in the fog computing environment. In this paper, we propose a novel fog computing model and offloading policy which can effectively bring the fog computing power closer to the mobile user. The fog computing model consist of remote cloud nodes and local cloud nodes, which is attached to wireless access infra-structure. And we give task offloading policy taking into account executi+on, energy con-sumption and other expenses. We finally eval-uate the performance of our method through experimental simulations. The experimental results show that this method has a significant effect on reducing the execution time of tasks and energy consumption of mobile devices.