The current research on the hydrodynamic derivatives in open water is relatively mature, but the research on the hydrodynamic derivatives under ice-water coupling is still in infancy. In order to solve the hydrodynamic derivatives of a ship in oblique motion after ice-water coupling under brash ice conditions, the DEM particle of the CFD software STAR-CCM+ was used to simulate brash ice particles. The momentum and energy exchange was performed to achieve the coupling effect between ice and water by turning on the two-way coupling mode under the DEM module. The small drift angles of 0°, 2°, 4°, 6°, and 8° were selected for the numerical simulation of oblique motion. The effect of the free surface was ignored. The lateral force and turning moment of the ship under open water conditions and brash ice conditions were calculated respectively. The hydrodynamic derivatives were obtained by fitting the dimensionless force and moment at each drift angle. Considering the randomness of the brash ice interference, the maximum and minimum values of the force and moment after the interference were used to solve the corresponding hydrodynamic derivatives, so as to form a hydrodynamic fluctuation interval, which can more accurately predictthe hydrodynamic derivatives of the ship under ice-water coupling. Calculation results show that the hydrodynamic derivatives of the ship at each part under open water conditions were not significantly different from those calculated by the statistical model. The end values of the fluctuation interval of some hydrodynamic derivatives after ice-water coupling were positive and negative. Hydrodynamic derivatives were more random under brash ice-water coupling.