The invention relates to an oil-based drilling fluid shear strength-improving agent with an aluminum phosphate structure and a preparation method thereof. The preparation method comprises the steps of firstly, enabling hydroxyl-containing unsaturated monomers, alkyl alcohol, phosphorus pentoxide and oil soluble unsaturated monomers to perform esterification and polymerization reaction under the condition that an initiator exists, so as to obtain a mixture of alkyl phosphate and a polymer with a phosphate ester structure; and then, with aluminium salt adopted as a cross-linking agent, carrying out coordination cross-linking on phosphate ester in the mixture, so as to obtain the oil-based drilling fluid shear strength-improving agent with the aluminum phosphate structure. The molecular weight of the polymer component in the shear strength-improving agent can be controlled, an intermolecular network structure is formed under strong coordination cross-linking action of phosphate ester and aluminium salt in the molecules, the dosage of a product is low, the shear strength improving effect is excellent, and the influence on plastic viscosity of a drilling fluid is relatively small.