A device with a wing-shaped curved surface was designed to separate impurities based on the principle of the Coanda effect. The device improved the operational efficiency and separation quality of the second-stage impurity separation device of a corn breeding plot harvester. Moreover, the device solved some problems of conventional devices for cleaning and separating impurities, such as large size, poor flow field stability, and long time to stabilize the flow field. The structural parameters of the device were determined by the theoretical analysis of the gas flow field and the simulation tests by using computational fluid dynamics and discrete element method. Particles in the flow field experienced complex forces, among which the trailing force was the most effective for separating small particles. The magnitude of particle trailing acceleration was mainly influenced by particle density and diameter. Therefore, two sets of single-factor tests were conducted to investigate the separation effect of the device on particle mixtures with different densities and diameters, respectively. By analyzing the experimental results of the single-factor tests, the relationship equations between the density particles and their streamline deflection angles and the relationship equations between the diameter particles and their streamline deflection angles were obtained. Finally, the bench test results showed that the wing-shaped curved surface impurity separation device had a feasible separation function for the light impurities in the mixture after corn threshing. The bench test data indicated that the impurity content of corn kernels was as low as 1.014%, and the operational performance index of this device met the expected design requirements.