When the natural frequency of the flexible manipulators is equal to the frequency of the mode force and the mode force is stronger, the dynamic singularity will happen. The structure parameter and kinetic parameter of the flexible manipulator are the factors which influence the natural frequency of the flexible manipulator. A method to avoid the dynamic singularity was given on the condition that the predetermined task is not changed. The method is that a mass can be attached to the existed flexible manipulator to change the frequency of the flexible manipulator, so as to avoid the dynamic singularity. A method to plan the mass and the position where the mass attached to the flexible manipulator was given. A space manipulator with three arms ( the last one is flexible) was simulated, and the method was verified to be effective. This method can also used in other mechanism system.