Ultrahigh-Quality microcavity with whispering-gallery-mode (WGM) is a new kind of resonator with high quality. The evanescent wave of WGM plays an important role in surface enhanced Raman scattering (SERS). In this paper we study the relationship between coupling coefficient and evanescent wave of the WGM and the coupling character difference between the microsphere and the microstadium. Based on the finite-difference time-domain (FDTD), we calculate the coupling coefficient of different substances (water, C2H5OH, CHCL3, olive oil, C6H6 and CS2) with different dielectric constant and the mode volume ratio of the evanescent wave in microsphere and microstadium of the above substances in near field. The results show that (1) the mode volume ratio of evanescent wave will decrease with the increasing of the coupling coefficient; (2) the microstadium has a higher coupling coefficient than the sphere one.