Low-density polyethylene (LDPE) is the basic material of the high-voltage direct current power cable insulation. The assisted electric field is a common way to regulate the microstructure of polymers, but its application in the field of electrical insulating polymers is rarely reported. To study the effect of the assisted electric field on the microstructure evolution and direct current (DC) electrical properties of LDPE, the LDPE treated with assisted electric field at the melting stage, cooling stage and the whole stage (i.e., the melting stage and cooling stage) are prepared together with the untreated LDPE, respectively. The effect of the assisted electric field applied at the different stages on the microstructure evolution of LDPE is characterized by the scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The DC electrical properties of the untreated LDPE and the LDPE treated with the assisted electric field are investigated via the breakdown strength, conductivity, space charge and surface potential decay measurements, respectively. The results show that, compared with the untreated LDPE, the LDPE treated with the assisted electric field at the whole stage has the smallest spherulite size and the largest spherulite number, followed by the LDPE treated at the cooling stage and the melting stage, respectively. The application of assisted electric field at different stages can significantly improve the DC electrical properties of LDPE. Compared with the untreated LDPE, the breakdown strength of the electric field assisted LDPE under the melting stage, the cooling stage and the whole stage increases, whereas the conductivity and space charge accumulation of the electric field assisted LDPE decrease greatly. The DC electrical properties of LDPE treated with the assisted electric field under the whole-stage are the best. Compared with untreated LDPE, the breakdown field strength of LDPE with whole-stage treatment can be increased by 35.8 %, the conductivity is decreased by 72.0 %, and the space charge accumulation is reduced by 20.2 %. More and smaller spherulites lead to the formation of more interface paths and introduce more deep traps, which contributes to the improvement of DC electrical characteristics of the electric field assisted LDPE. This work provides a new idea for the improvement of DC electrical properties of polymers.