Quasielastic scattering excitation function at large backward angle has been measured for the weakly bound system ⁷Li + ¹⁵⁹Tb at energies around the Coulomb barrier. The corresponding quasielastic barrier distribution has been derived from the excitation function, both including and excluding the α particles produced in the reaction. The centroid of the barrier distribution obtained after inclusion of α particles was found to be shifted higher in energy, compared with the distribution excluding the α particles. The quasielastic data, excluding the α particles, have been analyzed in the framework of continuum discretized coupled-channels calculations. The quasielastic barrier distribution for ⁷Li + ¹⁵⁹Tb, has also been compared with the fusion barrier distribution for the system.