In this paper, when two robots rigidly grasp an object to contact the gurround, using the simplified model of the two symmetric coordinated robots, we analyze the effect on the stability of the internal force and external force symmetric control system due to the unmodeled factors: (1) the equivalent motor dynamic of robot, (2) the stiffness of the object grasped by two robots, (3) the dynamic of the object, (4) the sampling control delay of the computer control, (5) the dissymmetry of two robots. Then the results of the simulations are given, which accord with the experimental results.