The theoretical natural frequency under free oscillation was computed based on finite element method and potential theory, which was validated by experiment with FFT. Under the filling level of0.25, 0.40 and 0.60 times of model tank's diameter, a series of sweep test on excitation frequency near theoretical natural frequency were carried out with external oscillation and same amplitude. The description on profile of free surface in forced oscillation was given. Results indicate that the sloshing started from 0.5 times of the theoretical natural frequency and the ending frequency of sloshing would decrease as filling level increasing. The relationship between theoretical natural frequency and resonant frequency in three different filling level was analyzed quantitatively with statistical values. As results, the resonant frequency exceeds the theoretical natural frequency by 31%, 13% and 7%, respectively.