Gas replacement method and gas replacement device before natural gas steel pipe fire operation

The invention relates to a gas replacement method and device before natural gas steel pipe fire operation. The gas replacement method comprises the following steps that S1, openings in the two ends of a pipeline are closed, and deflation valves at the two ends of the pipeline are opened; s2, a deflation valve at one end of the pipeline is filled with inert gas, compressed air continues to be filled after the inert gas is filled for one kilometer, and the gas pressure is maintained to be 50 MPa; s3, a connecting pipe is arranged on a gas outlet of the deflation valve far away from the end filled with the inert gas, and the end, far away from the deflation valve, of the connecting pipe is ignited to ignite the discharged natural gas; and S4, continuously filling compressed air until the gas exhausted from one end, far away from the deflation valve, of the connecting pipe is not ignited any more. When the inert gas is used for driving the natural gas out of the pipeline, the inert gas is used for driving the nat