A modular submersible repairing system includes a working unit having a tool module capable of repairing structures in a reactor. A scanning/pitching module is capable of being selectively connected to or disconnected from the tool module, and is provided with a scanning/pitching shaft for scanning or pitching the tool module. A submersible fan module is capable of being selectively connected to or disconnected from the scanning/pitching module. A first buoyant module keeps an orientation of the tool module. A base unit includes a manipulator module internally provided with an actuator driving mechanism. An adsorbing module is capable of being detachably mounted on the manipulator module and a wall. A second buoyant module keeps the orientation of the manipulator module. The scanning/pitching module and the manipulator module are provided with a submersible connecting device capable of being operated in water for engagement and disengagement.