The invention belongs to the field of thermally-conductive and flame-retardant materials and discloses a high-thermal conductivity flame-retardant graphene flexible film and a preparation method thereof. The high-thermal conductivity flame-retardant graphene flexible film is characterized in that a reducing agent with flame retardance is introduced into the surface of a graphene oxide sheet in a manner of functionalization or grafting, graphene oxide is reduced, and then a filler is added and assembled into the film; and the mass ratio of the reducing agent with flame retardance to the graphene oxide is 1:(1-10), and the mass ratio of the reducing agent with flame retardance to the filler is (1-1.2):1. The thermal conductivity coefficient of the high-thermal conductivity flame-retardant graphene flexible film in the plane direction can reach 15 W m<-1>.K<-1>, the thermal conductivity coefficient of a vertical plane can reach 0.8 W m<-1>.K<-1>, and the tensile strength is 35 MPa; and inaddition, the high-thermal conductivity flame-retardant graphene flexible film is high in flame retardance and free from thermal release below 500 DEG C, and when the maximum thermal release temperature is 540 DEG C, the thermal release peak vale is below 37 W.g<-1>.