Deep mines contain abundant geothermal energy. As a solid heat storage material, the backfill body near the surrounding rock and stope can continuously absorb heat. The phase change material (PCM) embedded in the tube-in-tube heat exchanger is more conducive to the accumulation of geothermal energy. In this paper, the backfill body with tube-in-tube heat exchanger is taken as the research object and its heat storage process is simulated by FLUENT. The influence of surrounding rock temperature, initial temperature of backfill body, airflow temperature and velocity in stope on the heat storage performance of backfill body is analyzed. The results show that adding PCM can increase the heat storage capacity and the average increment is 155.2 kJ within 10 h compared with ordinary backfill body. The total heat storage capacity always increases over time with an increment about 90% occurring within 5 h. This paper provides a theoretical basis for the study of the heat storage performance of the backfill body under different working conditions in mines, and also lays a foundation for the efficient accumulation of geothermal energy and the exploitation of deep geothermal energy.