The invention discloses a bismuth / carbon nitride composite catalyst, and a preparation method and applications thereof. The preparation method comprises following steps: 1, C3N4 precursor and Bi2O3 precursor are mixed at a certain ratio so as to obtain a Bi2O3 / C3N4 powder; 2, the Bi2O3 / C3N4 powder and KI are mixed at a certain ratio, an obtained mixed material is dispersed in water, stirring is carried out continuously after uniform mixing, a diluted sulfuric acid solution is added dropwise with stirring, after reaction, centrifugation, water washing, drying, and grinding are carried out so asto obtain a Bi2O3-BiOI / C3N4 powder; 3, the Bi2O3-BiOI / C3N4 powder is dispersed in methanol, ultraviolet irradiation is carried out under anoxic conditions, and centrifugation, water washing, drying,and grinding are carried out so as to obtain Bi@Bi2O3-BiOI / C3N4. According to the preparation method, synchronous introduction of C3N4 in preparation process of Bi2O3 is capable of obtaining Bi2O3 / C3N4; Bi2O3-BiOI / C3N4 is prepared under KI etching effect; in-suit ultraviolet reduction of a part of Bi plasma is realized so as to realize improvement of visible light responsiveness, and Bi@Bi2O3-BiOI / C3N4 is obtained.