Abstract: To achieve the conversion of data between wind tunnel and water tunnel experiments, this paper conducts a study on the similarity of fluctuating pressure on the surface of a cylinder in water and air based on a numerical method by using large eddy simulation. Firstly, the accuracy of the numerical simulation method used in this paper is verified by comparing the spectra of far-field sound pressure levels in air and water around a cylinder. Secondly, for the inflow at a Mach number of 0.2, the effect of air compressibility is investigated. It is found that at the current Mach number, the compressibility of air has a minor effect on the radiated sound. Then, from the perspective of energy, the power spectral density of the fluctuating pressure on the cylinder surface is nondimensionalized, and the similarity is analyzed. Spectrum analysis of the surface fluctuating pressure at different wall positions of the cylinder in both water and air media is conducted for Reynolds numbers ranging from 2.0 × 10⁴ to 1.0 × 10⁵. The results show that when the Reynolds number is greater than 6.0 × 10⁴, the surface fluctuating pressure on the cylinder in both water and air media exhibits good similarity.