The invention discloses supported graphene-based nano composite and a preparation method and application thereof. Protonated carbon nitride is supported as a nano bridge to graphene-based nano material to obtain the composite. The preparation method comprises the steps of (1) preparing graphene oxide water suspension with graphite oxide solid under ultrasound; (2) adding protonated carbon nitrideinto the graphene oxide water suspension for uniform ultrasonic treatment; (3) adding FeCl3 6H2O and polyvinylpyrrolidone into the suspension of step (2), and stirring well; (4) performing hydrothermal reacting quickly; (5) washing, and drying to obtain the supported graphene-based nano composite. The supported graphene-based nano composite is prepared by scientific integration of nano metal oxidein-situ growth, graphene oxide synchronous reduction and protonated carbon nitride and graphene synchronous self-assembly; the synthetic steps are simple and efficient; the supported graphene-based nano composite is easy to prepare on large scale and is particularly applicable to electrochemical catalytic detection and analysis for nitrites.