The invention relates to a magnetic rheological polishing fluid and a preparing method for the fluid. The magnetic rheological polishing fluid comprises by volume magnetic particles 30-40%, pure water 45-55%, additive 5-10%, surface active agent 3-5% and polishing powder 5-8%. The preparing method comprises: the magnetic particles are surface cleaned and activated, the surfaces of the magnetic particles are treated by a hi-speed ball pulverizer. The magnetic rheological polishing fluid prepared by the invention is of hydrolyzation function. The preparing method is simple. The fluid is of a good resistance to settlement and conglobation, low zero-field viscosity, high polishing efficiency, good effect, long service life, and is applicable for optic polishing. The invention solves such problems of prior magnetic rheological polishing fluid as high viscosity, weak magnetizing strength, liable to caking, not stable in settling and conglobation, liable to rust, short service life and low efficiency. The magnetic rheological polishing fluid prepared by the invention is of high efficiency, and can facilitate the surface finish of a k9 glass to increase from 400nm to 0.6 nm in 30 minutes. The invention can be widely used in the super surface polishing of optic glass.