In this paper,by taking advantage of the micro/nanofiber′s strong ev anescent field and the plane grating′s refractive index periodically distributed properties,a refractive i ndex sensor based on a composite waveguide composed of a micro/nanofiber and a plane grating is innovatively desi gned.And then,by using the finite difference time domain (FDTD) method,the optical and refractive Index sen sing properties of the composite optical waveguide are studied in simulation.The results show that the center wav elength of waveguide′s transmission spectrum is very sensitive to the surrounding mater ial refractive index variation and the refractive index detection sensitivity is 346nm/RIU (refractive index unit).