Abstract: Oxidized sodium alginate (OSA) is a type of biomass-derived chrome-free tanning agent with good tanning performance. Exploring the mass transfer behavior of OSA in leather during tanning process can provide reference for the design, preparation and application of this type of tanning agent. Herein, highly purified N-methylisatoic anhydride (MIA)-labeled OSA (MIA-OSA) was successfully prepared through MIA labeling reaction followed by column chromatography purification. The labelled product exhibited high fluorescence intensity for a long time (one month) and at pH 7-8, and its molecular weight and distribution were similar to those of initial OSA. MIA-OSA was then used in the tanning of cattle hide. The mass transfer and the distribution of MIA-OSA in hierarchical layers of leather were investigated. Results indicate that MIA-OSA can penetrate and evenly distribute in the cross-section of leather after tanning for 1.5-2 h. As a result, the visualization of mass transfer of OSA tanning agent in leather during tanning can be achieved by using this fluorescence tracing technique.