Based on the kinetic model of underwater grid fins during the procedure of dynamic expansion, a prediction method was developed, which took the motion parameters at typical expansion points as the design input. The hydrodynamic moment used in the kinetic model was fitted by the values obtained in steady hydrodynamic conditions with specific expansion angles. A modified factor in consideration of the relative velocity was included as well. By comparing with the results obtained by unsteady numerical simulations and the underwater vehicle experiment data, the prediction method is verified, which can provide a design reference for the optimum proposal of the underwater grid fins and the analysis of nonsynchronization expansion.