The unsteady oscillatory characteristics in supersonic cavity are numerically studied in the steady inflow condition. 5 order WENO scheme and 3 order Runge-Kutta algorithm are used. Through numerically calculating and analyzing the pressure changing procedure, sound pressure level, oscillatory frequency in 2D and 3D cavity, it shows that there is typical pressure oscillatory in cavity. The oscillatory has approximate cycle, but the law of cycle is very complex. 3D cavity exerts little effect on the main oscillatory frequency, but the oscillatory intensity is quite different in 2D and 3D.