The invention relates to a preparation method of a TiO2-loaded composite photocatalyst. The method comprises the following steps: subjecting a mixed solution of a soluble zinc salt, 2-methylimidazoleand a first organic solvent to a heated reaction in an autoclave, subjecting 2-methylimidazole to deprotonation, self-assembling ZIF-8 from 2-methylimidazole and zinc ions, carrying out filtering, washing and drying, and then, carrying out grinding, so as to obtain ZIF-8 powder; subjecting a hydrolyzable titanium source to a hydrolysis reaction with water in a second organic solvent, so as to obtain a nano-titania containing first solution; concentrating the first solution to improve the concentration of nano-titania sol, so as to obtain a second solution; adding the ZIF-8 powder into the second solution, carrying out thorough mixing through ultrasonic oscillation, and carrying out filtering, washing and drying, thereby obtaining the TiO2 / ZIF-8 composite photocatalyst.