The invention belongs to the technical field of high-grade fireproof materials for metallurgical, colored, glass and cement kilns, and preparation thereof, and particularly relates to a method for preparing a non-burned modified high-purity magnesium aluminum spinel composite brick through microwave drying. According to the non-burned modified high-purity magnesium aluminum spinel composite brick, high-purity magnesia with a magnesium content of 99% is adopted as a raw material, the magnesium aluminum spinel composite brick is modified by using a complexing magnesium-aluminum cementing agent and a rare earth oxide, the preparation process uses a microwave drying technology, the microwave drying is designed into two controlled sections, the microwave power is controlled at 250-500 kw, the first section is microwave drying and the drying time is 10-20 min, and the section is the co-drying of microwave and recovery hot wind gas flow and the drying time is 10-20 min. According to the present invention, the preparation method is the high-efficiency and low-cost clean steel production technology, and has advantages of no requirement of high temperature firing, uniform heating, rapid dewatering, low raw material cost, excellent product performance, short preparation period, carbon-free and chrome-free emission, no pollution, zero discharge, and the like.