Through the process of resource distribution and user request, this paper proposed a user utility function for tasks in the cloud computing environment. In this model, the risk is invoked to quantize the loss of the services and resources in the un- authorization access. Based on risk, this paper proposed a user expected utility function. Finally, the simulation experiment demonstrates the relation among the tasks quantity and the users' request, time, and cost. Through the experiment, a perfect value exists for the task quantity, and near this perfect value, the users' utility can be optimized.