The invention relates to a method for treating medium- and low-grade bauxite to produce metallurgical-level aluminum oxide by a one-step andradite alkali heat method. The method comprises the following steps of S1, crushing and finely grinding the medium- and low-grade bauxite into mineral powder; S2, mixing and preparing the mineral powder, sodium ferrite or calcium ferrite, activated lime and circulating mother liquor into raw-material mineral pulp; S3, subjecting the raw-material mineral pulp to an alkali heat dissolving-out reaction; S4, diluting dissolved-out mineral pulp, and subjecting diluted liquor to liquid-solid separation; S5, introducing a carbon dioxide gas into a low-molecular-ratio sodium aluminate solution to carry out carbonation decomposition, so as to obtain crude aluminum hydroxide and carbonation-decomposed mother liquor; S6, subjecting the crude aluminum hydroxide to bayer-process treatment, so as to obtain the metallurgical-level aluminum oxide. According to the method provided by the invention, through adding the lime and the sodium ferrite or the calcium ferrite in a dissolving-out procedure, andradite-type slag instead of conventional dissolved-out slag is generated in the dissolving-out procedure; the dissolving-out rate of the aluminum oxide can be improved greatly; meanwhile, the metallurgical-level aluminum oxide is prepared; the method has the characteristics of being short in flow path and high in efficiency; the zero release of a material is realized.