Based on the experiments of the sandy soil penetration, a semi-cylindrical sandy soil bed in 3D was established, and the simulation with discrete element method for the penetration was investigated. According to the studies on the experiments and simulations, the analysis of influencing factors to the cone index of soil was discussed. And the result showed that the vertical resistance of the penetrometer, cone index and the normal force on the semi-cylindrical wall all increased as the increasing of the penetration depth. And the cone indexes of the narrow and big particle soil bed are much larger than the wide and small particle beds. Besides, the normal force on the semi-cylindrical wall of the big particle bed after penetration is larger than the small particle one. Only after the penetration velocity exceeded 0.2m/s would cone index increase obviously because of impact force from the penetrometer on the particle bed. Finally, according to the displacement of particles, the particle bed after penetration was divided into four zones, and the influencing area could be defined easily, which provided theoretical foundation for determination of distance between measurement points in situ test of soil compaction.