According to the problem of autonomous terrain following in unknown environment for UAVs, a novel terrain following method was introduced based on the skyline of the forward-looking image sequences. And the guidance law of adaptive angle method was designed in detail, where the forward terrain information was detected by the passive images in real time, so the digital map and the high precision navigation requirement were reduced. The 6DOF simulation results demonstrate that the method is better than the traditional method based on digital map, the average terrain following height from the ground can be reduced by over thirty-five percent, so the method can improve the penetration performance significantly.