The invention discloses a method for preparing magnesium oxide from asbestos tailings. The method comprises the following steps: 1) drying and calcining activated asbestos tailings to obtain calcinedtailing powder; 2) adding the calcined tailing powder into an ammonium salt solution, leaching under the conditions of decompression, water bath heating and stirring to obtain magnesium salt suspension liquid and ammonia gas, and absorbing the ammonia gas with water to obtain ammonia water; 3) filtering the suspension liquid to obtain a magnesium salt leaching solution; 4) reacting the magnesium salt leaching solution with the ammonia water to obtain magnesium hydroxide precipitate and an ammonium salt solution, calcining the magnesium hydroxide to obtain the magnesium oxide, and introducing the ammonium salt into a next cycle for recycling. According to the process, high-purity magnesium oxide can be prepared from the asbestos tailings without consumption of other raw materials; the method has the advantages of high yield, low cost and low energy consumption.