The invention relates to the technical field of ultraprecision machining, in particular to a magneto-rheological polishing device and polishing method for a dynamic magnetic field. The magneto-rheological polishing device for the dynamic magnetic field comprises a base, a polishing disk, a dynamic magnetic field generation assembly, a workpiece clamping assembly, an ultrasonic assembly, a first driving assembly and a second driving assembly, wherein the workpiece clamping assembly is mounted on the base and is arranged above the polishing disc, the polishing disc is connected to the first driving assembly, the dynamic magnetic field generation assembly is connected to the second driving assembly and is arranged below the polishing disc, the first driving assembly and the second driving assembly are mounted on the base, the ultrasonic assembly is mounted on the lateral part of the polishing disc, and a magneto-rheological polishing solution is contained in the polishing disc. A magneticchain string is rearranged by means of the dynamic magnetic field, so that a grinding material is updated and self-sharpened. A chemical reaction is fuller by means of an ultrasonic assistant chemical reaction. As a clamping disc revolves and rotates, a workpiece is in full contact with the magneto-rheological polishing solution, so that the polishing efficiency is improved.