Low-density polyethylene (LDPE) is the basic material of the high-voltage direct current (DC) 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. In order to study the influence of the assisted electric field on the microstructure evolution and DC electrical properties of LDPE, the LDPEs without and with being treated with assisted electric field are prepared in the melting stage, cooling stage, and the whole stage (i.e. the melting stage and cooling stage), respectively. The influence of the assisted electric field applied in 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 treated LDPE are investigated via measuring their breakdown strengths, conductivities, space charges and surface potential decays. The results show that, compared with the untreated LDPE, the LDPE treated with the assisted electric field in the whole stage has the smallest spherulite size and the largest spherulite number, followed by the LDPE treated in the cooling stage and the melting stage. The assisted electric field applied in different stages can significantly improve the DC electrical properties of LDPE. Compared with the untreated LDPE, the LDPE treated in the melting stage, the cooling stage and the whole stage increases the breakdown strength but greatly reduces the conductivity and space charge accumulation. The DC electrical properties of LDPE treated with the assisted electric field in the whole-stage are the best. Compared with untreated LDPE, the LDOE treated in whole stage increases the breakdown field strength by 35.8%, reduces the conductivity by 72.0%, and the space charge accumulation by 20.2%. More and smaller spherulites lead to the formation of more interface paths and introduce more deep-traps, which contributes to improving the DC electrical characteristics of the electric field assisted LDPE. This work provides a new idea for improving the DC electrical properties of polymers.