Abstract: The supercavitating flow hydrodynamics of grid fins of flat plates was addressed, and the effects on the grid fins’ hydrodynamic performances by different parameters including the grid fin plate number, the spacings between plates, the plate thickness, the attack angle and the cavitation number, were analyzed. A new hydrodynamic model for supercavitating grid fins was built and validated with experimental results. The hydrodynamic interference mechanism of the cavitating flow between the plates was explored and the experimental phenomena was explained. The results given by the proposed model with the numerical and experimental simulation show that, although the increase of the grid fin’s plate number promotes the lift force and drag force, the consequent decrease of the plate spacings, even down to the cavity thickness, will cause severe cavitating interference to weaken the hydrodynamic performances of the grid fin. Therefore, for a grid fin with certain sizes, the plate number has an optimal value.