The invention relates to a heat supply network water replenishing system for thermal power plants, in particular to a novel heat supply network water replenishing system. The system comprises high-tension heat supply network heaters and low-tension heat supply network heaters, and the high-tension heat supply network heaters and the low-tension heat supply network heaters are in parallel arrangement. Heating steam pipelines are connected to the high-tension heat supply network heaters and the low-tension heat supply network heaters respectively, two branches are connected to each of the high-tension heat supply network heaters and the low-tension heat supply network heaters, every two branches are merged into respective main pipes and then connected into a circulating water forebay and a water replenishing pump respectively, and the water replenishing pump receives water from a chemical water replenishing pipeline which is boosted through the water replenishing pump prior to merging into a heat supply network circulating water return pipeline. Compared with the prior art, the heat supply network water replenishing system has the advantages that heat supply network emergency drain water or heat supply network flushing water can serve as replenished water to be recovered to the heat supply network circulating water pipeline, and accordingly maintenance cost in abnormal operation period of the heaters and in commissioning test of the thermal power plants can be reduced.