In medical applications where quick and real-time detection of viruses, bacteria, antigens, etc. is essential, the use of highly sensitive and label-free optical biosensors has received considerable attention. The high sensitivity of these optical biosensors is due to the low amount of light loss. Among the various optical biosensors, Whispering Gallery Modes (WGM) micro-resonators have been extensively studied due to their ultra-high quality factor, small mode volume, and a strong evanescent field. These remarkable features have led to the countless use of the highly sensitive sensors. In particular, the toroidal shape of these high-quality micro-resonators takes up less space because of its more compressed surface compared to the spherical-structured micro-resonators. Therefore, they are more integrated than the spherical structures. In this research, by using two-dimensional simulation with finite element method in Comsol Multiphysics software, we intend to improve the surface performance of the biosensor by adding a thin polymer layer, which as a result strengthens and increases the efficiency of the biosensor. For this purpose, four different types of materials including silicon nitride (Si3N4), titanium dioxide (TiO2), and two types of polymers including polystyrene (PS) and polymethyl methacrylate (PMMA), have been used as a thin coating layer on the surface of the biosensor. This research has been done in two wavelengths of 850 and 980 nm, and finally the best performance of each material with the related specific thickness is determined on the surface of the biosensor. In addition, in this research we have also measured the sensitivity of this biosensor by adding a biological element of Influenza virus type to the micro-resonator based on whispering gallery mode.