Moonpool is just like a cave-in cavity with a free surface.With the help of empirical mode decomposition (EMD) method this experiment wasconducted in a towing tank. The key part of the EMD method is that complicated data set can be decomposed into a finite and oftensmall number of `intrinsic mode functions (IMF)'. By a comparative method, thedata were collected with and without the moonpool in all conditionsseparately. With the empirical mode decomposition, theintrinsic mode functions of the data, and then the PSDspectrum of each IMF are obtained. From the difference of each PSD spectrum betweenthe results with and without moonpool, the data for the flowover the moonpool can be obtained.For the moonpool, the fluctuating force consists of the water oscillation andthe shear layer self sustained oscillation. The frequency of the wateroscillation is determined by the shape and size of the moonpool. Thefrequcney of the shear layer self sustained oscillation is also related withthe flow velocity over the moonpool, which will change with thevelocity. When the two frequencies are close to each other, a resonance willhappen. In this experiment, when the Froude number falls within therange of 0.26 to 0.39, theresonance phenomenon is observed. It appears that the water in themoonpool moves upand down in the vertical direction. This is so-called the ``Piston'' phenomenon.When the Froude number is greater than 0.52, there is no resonance phenomenonany more. In this paper, under some assumptions and the potential flow theory,the resonance frequency of the water in the moonpool is calculated,which is very close to the experiment result.In a resonance, the fluid fluctuating force in the moonpool isa harmonic force. The frequency is the same as the resonance frequency andthe amplitude is great. When there is no resonance, the fluctuating force can be obtained with the EMD method, which consists of the shear layer oscillation and the water oscillation.The wall pressure is also measured in this investigation. The wall pressureof the moonpool is found quite different not only in value but also in distribution in the circumferential direction and the vertical direction,whether in resonance or not.