Abstract: This study systematically investigates the thermodynamic and kinetic mechanisms of nitrogen absorption and desorption in molten steel through experiments on X17CrNi16-2 stainless steel in a vacuum tube furnace. Thermodynamic analysis shows that within a 0.10 MPa range, the experimental nitrogen solubility aligns well with predictions based on Sieverts’ law and the Wagner model. The kinetics of nitrogen absorption and desorption were investigated by selecting two representative regions (top and bottom) of the molten steel, and a kinetic analysis method combining integral fitting, parameter consistency testing, and rate analysis was proposed to identify transitions in rate-limiting steps and resolve ambiguities when different reaction orders yield similar fitting quality. The absorption mechanism is as follows: nitrogen mass transfer controls the absorption rate in the bottom region, with a mass transfer coefficient of 0.0486-0.0505 cm/s. In the top region, interfacial chemical reactions dominate during the early stage (0-30 minutes), with a measured reverse reaction rate constant of 0.4464-0.5310 cm/(s·%). In the subsequent stage, the rate is governed jointly by interfacial reaction and mass transfer. During desorption, microscopic observation revealed bubble formation within the first 5 minutes. By comparing the apparent mass transfer coefficients and interfacial chemical reaction rate constants between absorption and desorption, a consistent enhancement factor of approximately 2.09-2.47 was obtained in the present small crucible melt pool. The top region is mainly controlled by interfacial chemical reactions, while the bottom region is dominated by nitrogen mass transfer, and the kinetic parameters, once corrected for the bubble effect, closely matched the absorption results.