The study of mode characteristics is a foundation for various applications of microcavity. The finite difference time domain(FDTD)technique is used to obtain the responce characteristics of cavity-mode spectra. The FDTD technique is demonstrated in the case of plane-parallel cavity,and the numerical results can coincide well with the theory results. The whisper gallery mode(WGM)characteristics of the plane-circular cavity are analyzed,and the results indicate that wavelength-scale cavities have good mode-selecting power. The detailed research on plane asymmetric resonant cavities composed of two arched boundary under different deformation parameter indicates that by changing the cavity shape the purpose of improving the performances of microcavities can be achieved. The results also show that it is a practical way to optimize the performance of cavities by optimizing cavity shape.