Relating to the field of semiconductor materials, the invention aims to provide a preparation method of a g-C3N4 quantum dot sensitized TiO2 nanotube. The method includes: adding a sulfuric acid aqueous solution into a melamine aqueous solution dropwise to prepare melamine sulfate, and conducting heat treatment; dispersing the product in an acid liquid to conduct ultrasonic treatment, performing standing in a dark environment, and washing, drying and grinding the product; then dispersing the obtained product in concentrated ammonia water, then carrying out hydrothermal reaction, separating and washing the product, and dispersing the product into anhydrous ethanol to obtain g-C3N4 quantum dots; reacting anatase-rutile mixed crystal type nano-TiO2 powder with an aqueous solution of sodium hydroxide, filtering and cleaning the product, performing drying, and then conducting soaking in an aqueous solution of hydrochloric acid; then dispersing the product into a hydrochloric acid water-alcohol solution, adding the g-C3N4 quantum dots and performing ultrasonic dispersion; conducting rotary evaporation treatment, then filtering the precipitate, and conducting washing and drying, thus obtaining a final product. The method provided by the invention improves the compounding ability of g-C3N4 with other materials, and the product has the advantages of good compounding uniformity and loading stability, large surface area and high quantum efficiency, and can increase the surface active sites and improve the efficiency of photocatalytic reaction.