Developing high-performance polyethylene-based insulating materials for high-voltage direct current (HVDC) application is still a great challenge owing to space charge effects especially under DC conditions. In particular, microscopic crystal-amorphous structures make the charge transport process more complex under coupled fields, i.e., electric, temperature, and mechanical fields. Here, we investigate the effect of heat treatment on the morphology and space charge behavior of low-density polyethylene (LDPE) under temperature fields ranging from 20 to 60 ℃ under a high external DC field of −100 kV/mm. The results show that better crystallization in LDPE was achieved with a lower annealing rate, and both the size and level of uniformity of spherulites increased. Positive and negative charge packets were observed and followed the field-dependent negative differential mobility (NDM) law, which were strongly affected by temperature. According to the change in trap property, the effect of crystal-amorphous regions is critical to long-range hole transport, because larger spherulites showed increased difficulty on interchain tunneling, reduced carrier mobility, and reinforced hole trapping.