Abstract: This paper analyzes the formation mechanism of composite foundation in the karst area, at the same time studies the strengthening mechanism of the CFG pile composite foundation by using the finite element software Plaxis, and discusses some problems in the application of CFG pile composite foundation by integrating correlative data, the main conclusions are: the CFG pile composite foundation in karst area has higher capacity and ability to effectively control the settlement; the pile bearing on the bedrock surface belongs to frictional end-bearing pile or end-bearing pile, so inclined rock face has significant impact on the bearing capacity of single pile, a suggestion that the calculation of single pile bearing capacity should be according to friction pile under permissive condition; the property of the overlaying soil changes greatly in karst area, the article 9.2.5about the bearing capacity calculation of the CFG pile composite foundation in the Technical Code for Ground Treatment of Buildings is worth discussing, and so is the article 9.2.8about the calculation of the composite layer compression modulus, it was suggested to use the weighted average f_sk of subgrade bearing capacity characteristic value withing the range of pile length to calculate the bearing capacity of composite foundation f_spk.