In order to improve hit potency of tank guns in sophisticated topography, this paper has researched the optimize method of tank guns with 3D geographic information. With the information of target position, the paper advanced an algorithm of optimal angle of fire based on GA. Then it solved out the CEP of the point of fall through interpolation in order to build the model of it. Ultimately, it advanced the model of alternative optimal fire point based on 3D geographic information. Meanwhile, we proved the feasibility and validity through analyzing a case. It turned out that this method has an accurate ballistic model and accomplishes option of fire points within 3~4s medially, which can be a tool of paramilitary decision-making.