We study periodic cavity flow at Re=15 000, with a high quality non-uniform mesh obtained by boundary-fitted coordinated method and extra quadtree encryption method based on structural grid. Fourier power spectra analysis and phase-space trajectory were applied to understand periodic evolution and kinetic energy distribution of the flow field. Fundamental frequency and corresponding period were determined. It was found that during periodic evolution process of the flow field, the primary vortex maintained a low frequency of kinetic energy, while high-order vortices near boundaries and corners showed high frequencies of kinetic energy. Benchmark solutions of two characteristic states in period evolution were obtained.