Abstract: A gas injection device, wherein comprising: a gas channel including an air inlet provided at a upper portion therein and a gas outlet provided at a lower portion therein; and a light channel including an incident light channel and a reflected light channel provided at each side of the gas channel separately, wherein gases arrives at a surface of a sample to be tested via said gas channel and flows out from a slit between said light channel, the gas outlet of gas channel, and the surface of the sample to be tested, and gases flow in a manner of laminar flow with the Peclet number of an air flow being larger than 1. The gas injection device can effectively prevent air from returning back to the measurement system.