The automatic assignment of user task is a key technology for increasing the running efficiency of workflow management system. Workflow engines are always obliged to assess a complex and confusing situation, identify the complex relationship of candidates and tasks, decide to assign a special task to the most appropriate candidates, and ensure workflow system executes efficiently. The effectiveness depends largely on the evaluation of the influencing factors. After systematically analyzing the influencing factors and their linguistic fuzziness of user task assignment, a sectional multilevel model of task assignment and a task assignment method corresponding to the multilevel model are put forward. Furthermore, the design method of all adaptive weighting factors in the task assignment model is also discussed briefly. Finally, the performance of the multilevel model for task assignment is given by comparing the simulation result of the multilevel model and other two task assignment methods.