A lithium-ion battery system and a control method for combined internal and external heating are provided. A battery is heated in a low-temperature environment through combined internal and external heating. The energy released during self-heating of the battery is fully used, and rapid heating of the battery in the low-temperature environment is implemented. A current adjustment module in a heating module is controlled to adjust a switch on-off frequency and a current on-off time during the heating, and loops with different heating resistances in a multi-loop heating film are selected through a resistance adjustment switch. In this way, target heating requirements of the battery are met, such as a high heating rate in a low-temperature environment, low energy consumption during the heating, and a small impact on battery life without jeopardizing safety during the heating.