In severe cold regions, cold accumulation is caused by long-term heating-only operation of medium-deep ground source heat pumps, with shallow reverse heat exchange. with extractable thermal power reduced annually. A 2D axisymmetric numerical model of a 2000 m coaxial-casing deep borehole heat exchanger (DBHE) is developed. Four scenarios are compared over 5 years to evaluate the heat transfer performance of 300 m shallow insulated backfill under natural recovery and seasonal solar thermal storage, and examine the storage temperature effects on system efficiency. Under natural recovery, the heat extraction rate is increased by over 2% with insulated backfill, yet surrounding rock thermal degradation cannot be fully prevented. At a storage temperature of 70 ℃, thermal degradation is effectively mitigated by solar storage, with cumulative heat extraction for conventional backfill increased by 11.09%. Accordingly, Since shallow thermal insulation reduces the heat storage power, insulated backfill is recommended for natural recovery, while conventional backfill is preferable for storage-assisted operation. Theoretical support is provided for sustainable medium-deep geothermal development in severe cold regions.