The invention discloses an online gas mixing device with a function of accurately regulating mixing ratios. The online gas mixing device comprises a first gas inlet pipeline, a second gas inlet pipeline, a mixer and a mixed gas outlet pipeline. A first manual regulating valve, a first pressure gauge and a first flow meter are sequentially arranged on the first gas inlet pipeline along a gas flow direction, and a second manual regulating valve, a second pressure gauge and a second flow meter are sequentially arranged on the second gas inlet pipeline along a gas flow direction; two gas inlet branch pipelines are connected with the second gas inlet pipeline by a three-way pipe. The online gas mixing device has the advantages that the online gas mixing device is reasonable in design, and more than two types of gas can be uniformly mixed with one another in the pipelines in gas delivery procedures; the gas input can be regulated according to the mixing ratios, the online gas mixing device is high in regulation precision, and good mixing effects can be realized; the manual regulating valves are good in high-temperature resistance and corrosion resistance, excellent sealing effects can be realized, the sealing is easy to regulate, and the manual regulating valves are convenient to maintain and replace.