The ensemble Monte Carlo method was used to calculate the time-variation of scattering mechanisms and the carrier nonlinear dynamics evolution of n-doped GaAs and InSb in the high terahertz (THz) field. The time information of the electrons scattering into the side valleys and that of the electrons relaxation back into the original energy valley was directly obtained. The carriers transient increase process was also traced. Meanwhile, it showed that the intervalley scattering is the main mechanism of GaAs, while the impact ionization is a key point for InSb in high THz field. Furthermore, the work discussed the influences of the two mechanisms on related physical quantities: average kinetic energy, average velocity, and material conductivity. It indicates that the two mechanisms lead to nonlinear effects and play inverse roles in the two materials. The response time of impact ionization in InSb is longer than that of intervalley scattering in GaAs. The results have some guiding values in THz modulation field.