The invention discloses a water-based magneto-rheological closed flexible polishing head based on normal force. The polishing head comprises a flexible cutting disc, a magneto-rheological fluid, flexible seal rings, a rotary table, a C-shape magnetic yoke, a shell, baffles, a magnetic coil, a motor, a magnetic shield, shaft couplings, a key, a shaft and abrasive particles. The abrasive particles are evenly distributed on the top surface of the flexible cutting disc; the flexible cutting disc, the flexible seal rings and the rotary table form a seal cavity, and the magneto-rheological fluid is placed in the seal cavity; a rotating shaft is arranged on the center of the bottom surface of the rotary table, blind holes are upwards formed in the center of the rotating shaft, the motor is connected with the shaft through the shaft couplings, and the shaft is connected with the blind holes through the key; the motor is fixed to the top surfaces of the baffles, the motor is separated from the C-shape magnetic yoke through the magnetic shield, the bottom surfaces of the baffles are in contact with the magnetic coil, and the magnetic coil surrounds the closed end of the C-shape magnetic yoke. According to the water-based magneto-rheological closed flexible polishing head based on the normal force, a polishing liquid circulating device is not needed, the size of a flow device is greatly lowered, and the production cost is lowered.