The invention relates to a dynamic numerical simulation method for an external flow field of a multi-element airfoil of an airplane. The method comprises the steps that 1, an airplane model with a leading edge slat, a trailing edge flap and the multi-element airfoil is established and imported into fluent software; 2, a vertical descending motion function of the airplane model is constructed and imported into the fluent software; 3, a turbulence model is selected; 4, simulation parameters are set, including fluid parameters, airfoil material parameters, work environment parameters, boundary condition parameters, a dynamic mesh updating model, a solution method and solution precision; 5, the fluent software is run, and a flow parameter variation diagram, including a speed, pressure and temperature variation diagram, of fluid around the airfoil in the vertical descending process of the airplane is acquired. Compared with the prior art, dynamic numerical values of the external flow field in the landing process of the airplane can be effectively acquired, and reference can be conveniently provided for assessment of rising and landing performance of the airplane and design of a high lift device.