A floating non-contact ultrasonic enhanced flexible sub-aperture polishing device and method are disclosed, an X-axis moving platform, a Y-axis moving platform, a Z-axis moving platform and a workpiece swinging platform in linkage control ensure normal lines of a tool and a workpiece are kept at an identical angle to realize sub-aperture processing; the ventilation main shaft acts air pressure on the ball spline's end portion to form an axial thrust to let the tool axially float; a dynamic balance among hydrodynamic pressure, air pressure and dynamic pressure is performed on the polishing liquid by rotation of the flexible tool; a tiny gap between the tool and workpiece is formed by elastic deformation of the flexible tool due to the dynamic pressure; a shearing force for removing materials is generated when the polishing liquid flows through the gap; a cavitation effect in the gap is formed by ultrasonic waves.