Complexity and uncertainty of service environment where mobile robots work greatly challenge the performance of robot task planning system. In this paper, an effective and flexible task planning system of mobile robots is designed in distributed service framework with Robot Operating System (ROS). The first part of this paper is the design of system architecture. Then, the logical description tool Planning Domain Definition Language (PDDL) is used to describe the task dynamically based on the designed knowledge base and distributed sensing. In addition, a flexible and stable planning module is designed which combines Forward-Chaining Partial-Order Planning (POPF) planner, post-process, action dispatcher and exceptions handling programs. Lastly, the experiment results in distributed service framework of the task planning system are presented, proving the feasibility of the design.