Two different kinds of composite phase change materials (PCMs)—a high thermal conductive 80 wt% paraffin wax (PW)/expanded graphite (EG) composite and a 75 wt% PW/silica aerogel (SA) with a low thermal conductivity—are prepared and they characterized the thermophysical parameters. Then, a numerical model of battery pack based on composite phase change materials coupled with polyimide (PI) electric heating films is established at −20 °C. The temperature of monitoring points set in model and maximum and minimum temperature of the batteries in the pack are measured during discharge at 1C and 2C. By comparing the battery pack filled with PW/EG composite and the pack consisting of PW/SA composite, we intend to choose an appropriate one of the two composite PCMs to improve the lithium-ion batteries performance at low temperature. The results indicate that in spite of a good heating performance in heating process, the PW/SA composite induces an even higher temperature difference over the battery pack. Although PW/EG composite causes a large temperature difference at the end of heating film heating, it can quickly restore the uniformity of the battery pack. The PW/EG composite plays a more important role in improving the performance of the lithium-ion batteries at low temperature.