When the image guided missile attacks on the surface of strong fortification horizontally, the smaller the impact angle is, the easier for the follow-on warhead to successfully follow. According to a simplified motion model of missile, the Lagrangian was adopted for deducing an optimal trajectory shaping horizontal attack guidance law with falling point and impact angle constraint. The vertical trajectory coordinate, inclination and overload controlled by guidance system and functions of guidance time, as well as the influence of remainder guidance time were studied. The research provides the theory reference for product development.