The invention provides a preparation method for mesoporous carbon nitride, an N-TiO2/g-C3N4 composite photocatalyst and a preparation method thereof. The provided preparation method for the N-TiO2/g-C3N4 composite photocatalyst comprises the following steps: S1, after adequately mixing concentrated hydrochloric acid and tetrabutyl titanate, marking obtained solution as a solution A; S2, after adequately mixing absolute ethyl alcohol, the mesoporous carbon nitride and a nitrogen-containing compound, marking obtained solution as a solution B; S3, slowly dropwise adding the solution A to the solution B, after stirring and mixing, marking obtained solution as a solution C; S4, transferring the solution C to a hydrothermal kettle, heating in 80-150 DEG C, to obtain a white precipitate; S5, filtering, washing, drying, and roasting the white precipitate obtained in the step S4, to obtain the N-TiO2/g-C3N4 composite photocatalyst. The provided N-TiO2/g-C3N4 composite photocatalyst with an excellent heterojunction nano-structure is capable of responding visible light, and reducing combination of a light-generated electron and a hole, and has excellent photocatalytic activity.