Abstract: The paper introduces the modification for nitrogen sealing system of expansion tank in heat medium heater in Xiongxian pumping station. Due to the long pipings, more joints, nitrogen source being simultaneously used by the nitrogen sealing system and nitrogen protection system, the original nitrogen sealing system often worked in a failure condition, which shortened the service life: of heating agent. In the light of these defects, a modification program has been implemented, and items includes: keeping the nitrogen sealing system separate from nitrogen protection system to shorten the pipings of nitrogen sealing system to cut down leakage points; substituting liquid seal for safety valve; connecting general pipings with welding way; reducing the pressure of nitrogen supply; plugging the corroson holes on the inner surface of flange with epoxy and adopting highly elastic sealing washers. The modified system shows that the nitrogen com-suption is declined and obvious economical benefits have been obtained.