The interactions between high-energy charged particles and spacecraft insulating materials can cause deep dielectric charging and discharging, leading to spacecraft anomalies. In this paper, we establish a unipolar charge transport physical model of deep dielectric charging, according to the charge distribution and energy deposition of incident electrons and nonlinear dark conductivity and radiation induced conductivity (RIC) of material. Under the irradiation of electrons with different energies (from 0.1 to 0.5 MeV), the charge transport process of low density polyethylene (LDPE) can be obtained through solving the charge continuity equation and Poisson's equation. The calculation results show that the maximum electric field decreases with the increase of radiation electron energy. When radiation electron energy is less than 0.3 MeV, the distribution of the maximum electric field is similar to the change of the electron beam density. When the electron beam density is more than 3×10-9 A/m2, the maximum electric field will be greater than breakdown threshold (about 2×107 V/m), and it has higher risk of electrostatic discharge (ESD). With the increase of incident electron energy, the critical electron beam density will increase. When the radiation electron energy is 0.4 MeV, the critical electron beam density is 6×10-8 A/m2. When the radiation electron energy is more than 0.5 MeV, it seems that no electrostatic discharge (ESD) will occur in a range from 10-9 to 10-6 A/m2. The physical model has the great significance for further studying deep dielectric charging, evaluating the charged degree of spacecraft in space environment and designing protection devices.