The invention relates to a preparation method of a modified alumina catalyst for the dehydration reactions of isobutanol. The preparation method comprises the following steps: (a) mixing an aluminum salt solution and an alkali solution; (b) subjecting the mixture obtained in the step (a) to an aging treatment to obtain pseudo-boehmite gel; (c) mixing the pseudo-boehmite gel obtained in the step (b) with amorphous silica-alumina, drying the mixture, and carrying out extrusion moulding to obtain a carrier; and (d) dipping the carrier obtained in the step (c) in a caustic alkali solution and a calcium salt solution, and performing drying and burning to obtain the modified alumina catalyst. The prepared catalyst is applied to dehydration reactions of isobutanol to prepare isobutene; and the conversion rate of isobutanol and selectivity of isobutene are both improved.