The field intensity will be strengthened with the electric field induced by space charges added on the applied electric field if the voltage polarity of DC cable is reversed

which is possible to cause partial discharge or dielectric breakdown. The transient variations of space charges and electric field during the process of polarity reversal is difficult to be revealed by the present measurement methods

especially the interaction processes between microscopic particles. In order to deeply study the effects of the polarity reversal on the insulating properties of HVDC cables

the dynamic characteristics of space charges in low density polyethylene（LDPE） are numerically investigated by the bipolar charge transport model. Meanwhile

the transient distributions of space charge and electric field in LDPE under several polarity reversals are investigated. The results show that the surface electric field is strongly strengthened under polarity reversal. The field variation is of the polarity effect

which is more significant under the anode transiting to the cathode. The density of trapped charges is much higher than that of mobile charges before polarity reversal. And the strengthened electric fields on the electrode surfaces under polarity reversal are dominated by the trapped charges in the vicinity of the electrodes. The results can provide a reference for the insulation design of high voltage DC cables

as well as the dynamic simulation of space charges in polymeric materials.