Abstract: In the hot work of long-distance gas pipeline in service which is shut down for inspection or tie-in in route change, gas in the treated pipeline section should be vented and then displaced by nitrogen, for purpose of work safety. The inner leakage of block valve in the pipeline may cause gas mixing when the natural gas is displaced by nitrogen, so that the natural gas cannot be completely replaced. In this case, the middle nitrogen injection method is proposed, namely, after the pipeline is vented to slightly positive pressure, an injection nozzle is opened without hot work at the connection location, and then the nitrogen is injected to displace gas from the pipeline center to both ends. The practical application in a longdistance gas pipeline in Shaanxi Province shows that, comparing with conventional nitrogen injection method, the middle nitrogen injection method can effectively avoid the risk of on-line hot work caused by the inner leakage of block valve in the pipeline, and consequently realize thorough displacement under safe hot work.