Abstract: In order to improve the detection sensitivity and increase the response speed of the biosensor, based on the theory of planar waveguide, the zone energies of the evanescent fields in the single mode and multimode of the planar optical waveguide were compared under the same condition by using the principle of the evanescent field applied to the biosensing field. It is proved that the energy of the single mode is higher than that of the multimode by VC and Matlab. The theoretical calculation indicates that the fluorescence efficiency of single mode is 5.83% and multimode is 1.75%. Therefore, the single mode of planar optical waveguide is a better alternative in the application of biosensors and the relative area.