We numerically demonstrate that whispering-gallery modes (WGMs) in a microsphere resonator with three layers of high, low and high refractive index (RI) are analyzed by using the finite difference time domain (FDTD) method. To make the light couple in and out of the microsphere, a phase matched waveguide is used to overlap the WGMs eva-nescent radiation field. By changing the gap between the microsphere and waveguide, the WGMs of two high-RI lay-ers are efficiently excited. The stored energy and the mode volume are optimized for sensing applications. The cou-pling structure reveals a good sensitivity of 38.29 nm/RIU (RI unit).