The invention relates to a preparation method for modified magnesium aluminum spinel having acidity and alkality. The method is characterized in that modification is performed on magnesium aluminum spinel, such that acidity and hydrothermal stability are increased so as to improve cracking performance and retain partial oxidation desulfurization performance; the addition amount of the modified magnesium aluminum spinel can be properly improved without great impacts on a reaction performance of a main catalyst; the synthesized magnesium aluminum spinel is added with a solution containing nano-molecular sieve cluster, the resulting solution is adjusted to a certain pH value, and the adjusted solution is placed in a reaction kettle to carry out crystallization, such that a layer of a primary or a secondary structure unit of the molecular sieve is deposited on the surface of the magnesium aluminum spinel so as to enhance acidity and hydrothermal stability of the magnesium aluminum spinel.