In order to improve the effective mode field area,reduce the optical power density and reduce the nonlinear effect in the fiber,a graphene metal plasma double core sensor is proposed,which can be applied to a plasma waveguide biosensor. The cross section of the side polished fiber contains two cores with refractive index n1=1.4478,and the polished surface is coated with 30 nm gold film. The gold film is coated with a 0.34 nm graphene layer. Through the study of effective mode index,leakage loss and effective mode field area of the dual core fiber,it is confirmed that the monolayer graphene provides the best sensitivity to dielectric changes. The experimental results show that the effective mode area growth rate of the basic core mold LP01 is faster than that of the high order core mode LP11,and when the core refractive is 1.4460,the wavelength is 600 nm,the effective mode area of the base mode is 275μm2,which is greater than those of other core refractive values under the same conditions. The large mode area is obtained,and the nonlinear effect is reduced. In medical equipments,laser weapons,remote measurement and control,optical fiber communication and other important fields have a broad application prospect.