Most existing researches on task planning in joint operation simulations are not based on a complete formal theory, thus can only support the description of low-level interactions among entities, but can not support the planning of multi-level organizational groups. The description logic of tasks is a theory for describing organization structures and collaborative behavior plans. This paper proposes a method named TPDLT (task planning based on the description logic of tasks), which provides a formal description of the task planning in joint operation simulations based on the description logic of tasks, and gives a definition of the completeness of task plans and a decidable theorem of the completeness under joint strategies. At last, a deduction algorithm for task planning is given. As illustrated by a case study, with the method TPDLT, the task interaction semantics in virtual groups can be described succinctly and accurately, and powerful service for the accomplishment deduce of tasks is provided.