The recovery of lithium resources from salt lake brine has become more and more important. Tri-n-butyl phosphate (TBP) is the most common extractant and has good ability to selectively extract lithium, thus, basic research on it should be carried out. In this study, the kinetics of lithium extraction by TBP in salt lake brine was investigated at 298.15K via a single rising drop technique. Parameters affecting the mass transfer mechanism, including the interfacial area, lithium concentration, iron concentration, TBP concentration, and temperature were studied. According to this research, we found that the process of extraction is controlled by the diffusion and the chemical reaction occurs at the interfacial area. The mechanism of the lithium extraction with TBP is revealed.