By integrating theoretical analysis with experimental demonstration, this study develops a teaching apparatus for investigating the laser-induced thermal lens effect based on spatial self-phase modulation (SSPM). The apparatus makes it possible to observe the dynamic evolution of SSPM diffraction fringes from formation to distortion and finally stabilization. The results confirm a linear dependence of the diffraction-ring number on laser intensity and further show that, under the same incident intensity, wavelengths closer to the material’s absorption peak generate more diffraction rings. The experimental teaching system is extended from the conventional NMP (N-methylpyrrolidone) suspension system to a safer tea polyphenol-ethanol system and to solid samples. The proposed apparatus not only broadens the application of thermal lensing in experimental teaching, but also offers a useful platform for further studies of thermal lensing in emerging materials.