Six optical microcavities with different refractive index distribution are studied using the finite-difference time-domain method (FDTD). The energy distribution and quality factors of different radial-order whispering-gallery modes are calculated. The effect of step-index distribution and graded-index distribution are analyzed. Then the methods of enhancing certain modes and improving microcavity performance are also discussed. These results provide reference for the further study and application of optical microcavity.