The invention discloses a preparation method and application of a carbon nitride/bismuth selenate composite material and belongs to the technical field of material preparation and photocatalysis. Thepreparation method disclosed by the invention comprises the following steps: adding a certain amount of urea powder into water, heating to a temperature of 100 DEG C to be dissolved, adding Bi2SeO5 powder under an ultrasonic action, stirring for 6 hours, cooling to room temperature, centrifuging and drying in a drying oven at a temperature of 80 DEG C, fully grinding the dried sample, and calcining in a tube furnace in a reducing atmosphere, thereby obtaining the g-C3N4/Bi2SeO5 complex. The preparation process is very simple, and organic surfactants or template agents and the like do not needto be added. Meanwhile, the equipment requirement in the synthetic process is low, and no other expensive synthesis equipment or high-temperature high-pressure reaction device is needed, which facilitates large-scale popularization. The composite photocatalyst is applied to visible-light selective organic synthesis.