Abstract: Numerical investigation of the coupling mechanism of an 80° swept delta wing in free-roll and freesideslip is presented, in which a coupling numerical method is employed by solving NS equations and rigidbody dynamic equations. Comparing with the characteristics of single degreeoffreedom wing rock, fiercely wing rock phenomena in combined freeroll and freesideslip are observed: at the same roll angle, larger angular velocity is achieved; at the same angular velocity, larger roll angle is achieved. The investigation shows: the asymmetric oscillation of vortices leads to the wing rock of slender delta wing in combined freeroll and freesideslip; sideslip is induced by the side component of normal force during wing rock; right sideslip during left rolling and left sideslip during right rolling are the coupling kinematic mechanism of slender delta wing in combined freeroll and freesideslip motion; with the impact of sideslip, that attack angle and sideslip angle lag behind the roll angle is observed, the asymmetric characteristics of flow structures and hysteresis effects are enforced during coupling wing rock.