The outside surface shape of the aerial conformal optical window is determined by the aerodynamic requirement and is often an asymmetric free-form surface which produces a large number of aberrations. The aberrations can be reduced by appropriate inner surface shape of conformal window after design and optimization. The characteristics of free-form surface which is applied to the conformal optical window are studied. The specific design method of aerial conformal optical window is given. The conformal optical windows are designed for corresponding medium-wave infrared optical systems which are placed behind them. And the design results show that the conformal optical windows through design and optimization with this method reduces the aberrations in large quantities produced by the free-form outside surface. And the conformal optical windows have small influence on the image quality of corresponding optical systems when these optical systems are placed behind the corresponding conformal optical windows.