Abstract: Cooperative coverage by multi-simple-robot is discussed.For online coverage planning,a Multi-robot Internal Spiral Coverage algorithm based on grid map is presented to overcome the sensing limitation of simple robot that can sense the environment only by contact sensors.The algorithm guarantees complete coverage by repeating covering portion of the environment and setting the GATE grids.The algorithm improves system stability in the sense that coverage can be completed if only one robot is not in catastrophic failure.Simulation experiment proves the feasibility of the algorithm.