To study nonlinear aerodynamics of merging vortex pair, a simplified model is employed to obtain merging at different separating distances by solving N-S equations in an LES turbulence model. Interactions of vortex merging, effects of separation distance on merging and control of vortex position by merging of vortex pair with different vortex strength ratios are analyzed with topological structure of streamlines. It is shown that merging distance increases nonlinearly with separating distance. Increasing separation distance decreases ability to control vortex position.