Taking the unserviceable Guangxi high-iron composite bauxite as the research object, a new three-step process is developed based on clarifying the occurrence forms of alumina in bauxite, the results indicates that they are gibbsite, boehmite, Al-goethite, hematite, anatase, quartz and so on. Among these, iron oxides includes 72.22wt% exists in the form of Al-goethite and 27.78wt% exists in the form of hematite, and the substitution rate of aluminium for iron in Al-goethite is 24.5 mol/mol; Alumina in bauxite exists in three forms, namely gibbsite, boehmite and Al-goethite, each accounting for 37.48wt%, 3.89wt%, and 42.46wt% of the total alumina. Firstly, extract alumina in the forms of gibbsite and boehmite under conditions of Na2O 100 g/L, reaction temperature 180℃, and reaction time 1 hour, which can be easily and completely dissolved, while Al-goethite mineral is difficult to transform under this mild conditions, as seriously affects the economy of this new process. In order to solve the problem of alumina dissolution in Al-goethite, low-temperature roasting is adopted to transform Al-goethite into hematite and activated alumina in order to solve the problem of aluminium leaching from Al-goethite based on the thermal transformation mechanism from goethite to hematite at a certain temperature. By roasting this iron-rich phase for 0.5 h at a temperature of 450℃, about 57wt% of the alumina in Al-goethite is released outside the hematite lattice, thus aluminium-iron separation and phase transformation are achieved. In the third step, the transformed minerals are dissolved again under the conditions of low alkali concentration (Na2O 100 g/L), reaction temperature of 220℃, and reaction time of 1 h. Under these conditions, the activated alumina outside the hematite lattice is dissolved. The total dissolution rate of alumina in the three-step process reaches 85.92%, and the iron oxide in the mineral is greatly enriched in the tailings after dissolution, and its iron oxide content reaches 83.06wt%, which can be used as raw material for ironmaking.