The invention relates to a method for preparing aluminum hydroxide by treating middle-low grade monohydrallite through an andradite one-step alkali heat method. The method includes the following steps that 1, middle-low grade monohydrallite is crushed and ground into mineral powder; 2, the mineral powder, sodium ferrite, active lime and circulating mother liquid are mixed to prepare mineral pulp as raw materials; 3, the mineral pulp as the raw materials is subjected to an alkali heat dissolving-out reaction to obtain dissolved-out pulp; 4, after being diluted, the dissolved-out pulp is subjected to liquid-solid separation to obtain a sodium aluminate solution and dissolved-out slag; 5, the sodium aluminate solution reacts with lime milk to obtain tricalcium aluminate pulp; 6, the tricalcium aluminate pulp is subjected to liquid-solid separation; 7, tricalcium aluminate is washed and subjected to reaction under the action of a sodium carbonate solution to obtain calcium carbonate; 8, the calcium carbonate dissolved-out pulp is subjected to liquid-solid separation to obtain calcium carbonate solid and a sodium aluminate solution; 9, the sodium aluminate solution is subjected to carbonation and liquid-solid separation to obtain carbonation mother liquid and aluminum hydroxide. According to the method, the dissolved-out rate of alumina can be largely improved, and meanwhile the slag is almost free of alkali.