The laser-induced thermal lens oscillation process, which can be generated in an organic solution by argon-ion laser irradiation is studied to investigate the possibility of its application for monitoring of tri-n-butyl phosphate (TBP) concentration in nuclear fuel reprocessing. The oscillation process is a nonlinear dynamical system whose states depend on three control parameters: laser beam power, depth from solution surface to a laser beam irradiation position, and concentration of solvent, i.e., TBP. From a series of experiments, it is found that a transition between different states is distinctly related to the concentration of TBP solution. From this result, a new on-line monitoring method of solvent concentration is proposed. This method indicates the technique’s potential as a viable on-line analytical instrument in solvent extraction processes of nuclear fuel reprocessing.