The invention relates to a preparation method of a modified alumina carrier. The preparation method comprises the following steps: (1) adding a surfactant and a precursor of modified aids, namely phosphorus and magnesium into a proper amount of deionized water, wherein the total amount of the modified aids is 5 to 15 percent of the amount of the deionized water and the mass of the surfactant is 1 to 10 percent of that of the modified aids; (2) performing dispersion, refining and homogenizing emulsification treatment on the obtained slurry to obtain slurry with the average granularity being 0.2 to 5 microns; (3) adding pseudo-boehmite and a nitric acid solution into the treated slurry, completely mixing and stirring, mixing and kneading until a plasticizer is formed; and (4) forming the obtained plasticizer, drying and roasting at 500 DEG C to 800 DEG C for 5 hours to 20 hours to obtain a phosphorus and magnesium modified alumina carrier material. According to the method provided by the invention, the dispersibility is high, distribution of a carrier acidic center and a hydrogenation catalyst active center can be effectively regulated, mutual coordination is realized and the performance of the catalyst is improved.