At present,the consolidation degree and strength of composite drainage foundation cannot be calculated and predicted from theory,which causes inconvenience to engineering design and construction.Aiming at this problem,we rationally simplify the boundary conditions of composite foundation,deduce the theoretical model of composite drainage,analyze this problem by the finite element software of PLAXIS 3D,contrast the results of PLAXIS 3D with theoretical,and give the irregular correction coefficient of drainage control area a recommended value.The results show that the theoretical model results are consistent with the numerical simulation results.For the irregular correction coefficient of drainage control area,the recommended value of a is 1.20 to 1.35 when the distance between sand piles is more than 3 m,and is 1.35 to 1.50 when the distance between sand piles is less than 3 m.