Geothermal re- injection is a powerful measure to realize the sustainable development of geothermal resources, which have been widely used in various countries of the world. It is of great significance for the protection of geothermal resources, the reduction of resource waste, the extension of pumping well life and the reduction of environmental pollution. Tianjin is one of the earliest regions to carry out geothermal re- injection in China. By the end of 2019, the recharge rate of fractured reservoir of bedrock in Tianjin had reached 79. 61%. The change of seepage field of geothermal reservoir under large- scale concentrated pumping and re- injection, and whether it will cause the decrease of geothermal reservoir temperature have become the front research topic of thermal reservoir system. Taking Dongli lake area of Tianjin as an example, this paper selects 1, 5- naphthalene sulfonate sodium as tracer, and carries out the tracer test of well group during the central pumping and re- injection in the heating season from 2018 to 2019. The results show that there are no obvious changes in geothermal well pumping (or re- injection) and water temperature during the heating period, and the water level shows no obvious changes except under the influence of normal pumping and re- injection. The tracer recovery rate is very low in the test, which shows that the hydraulic connection between the pumping wells and the re- injection wells is poor, and the maximum flow velocity in the limited dominant channel is 448.42 m/d. The direction of the dominant channel is mainly concentrated in the north- east direction, and is consistent with the direction of Cangdong fault and its secondary fault, which is the main thermal- controlling fault in the area. Under the present exploitation and utilization mode, the temperature of the thermal reservoir will not change significantly. These conclusions are of practical significance for guiding the scientific development and utilization of carbonate reservoirs in the buried hills of north China.