Triboelectric nanogenerators (TENGs) are widely applied to latest nanogenerators which are based on contact electrification (CE). In order to predict the working process of TENGs effectively, this paper proposes a dynamics model to simulate the dynamic mechanism of CE. Taking Fe and polytetrafluoroethylene (PTFE) as contact materials, the field emission under a strong electric field could occur when the interface distance is small enough. The transferred charges in steady state and the differences of interface barrier are calculated by self-consistent field (SCF) method. The molecular dynamics (MD) simulation is carried out under the contact pressure and electric field. The structure results of MD simulation and the electric fields obtained by SCF calculation are the input parameters for each other which run alternately. According to dynamic interface distances and the differences of interface barrier, the dynamic transferred charges can be finally predicted by tunnel current density. The results reveal that the amounts of transferred electrons are positively corresponded to the external pressure and the energy conversion efficiency will reach the extreme value when the pressure is near 130 MPa.