The non-equilibrium state between magnons and phonons is the key to understand the spin-caloric phenomena. We developed a unique optical reflectometry technique to spatially resolve Kerr angle ( theta K) and optical reflectance ( R) in a magnetic insulator-thulium iron garnet (TmIG). The TmIG was subjected to a thermal gradient to estimate populations of thermally excited magnons and phonons through the variation of theta K and R. The results showed that the spatial gradient of theta K is different from that of R, indicating the non-equilibrium state between magnons and phonons. Particularly, the characteristic decay length of theta K was significantly influenced by the heating power and the magnetic field, suggesting non-linear magnon scattering in a high magnon density regime. Our work not only provides a scheme to investigate the spatial profiles of magnons and phonons but also reveals the magnon-phonon non-equilibrium in TmIG. Hence, this report will stimulate further studies based on magnon-phonon non-equilibrium such as a transverse spin Seebeck effect and Bose-Einstein condensation.