Numerical simulation is a good method for studying some academic problems in the engineering. Laboratory triaxial tests of composite soil were used widely in order to understand the stress-strain relationship. But the deformation of composite soil in laboratory triaxial tests is difficult to measure in some situation. Numerical simulation on laboratory triaxial test of composite soil with roots is carried out to see how the deformation of composite soil developed under the pressure on the specimen. Drucker-Prager constitutive model was used in the numerical simulation of finite element analysis. The results show that the numerical simulation of finite element method using Drucker-Prager constitutive model is suitable for studying the deformation development of composite soil with roots. The research results are important for understanding the deformation development of laboratory triaxial test of composite soil with roots.