<p>This study proposes an unbonded near-surface-mounted (NSM) timber reinforcement technique for in-service earthen buildings with pre-existing seismic damage. The proposed technique follows the principle of “repairing the old as original” and “strengthening with original materials”. Quasi-static tests were conducted on adobe walls in both unreinforced conditions and reinforced conditions with varying pre-existing seismic damage levels. The damage patterns and seismic performance of the walls after reinforcement were analyzed, focusing on the influence of the pre-damage level on the effectiveness of reinforcement. Based on further experimental data collection, the energy dissipation mode of this technique was compared with that of bonded NSM technique, thereby clarifying its seismic mechanism. The results show that although the unbonded NSM timber reinforcement does not change how the walls fail by diagonal shear, it effectively suppresses through-crack propagation through a spatial confinement effect formed by the vertical and horizontal timber framing combined with through-bolts. The lighter the pre-existing seismic damage is, the more evenly distributed the cracks become. The technique provides only a limited improvement in wall load-bearing capacity (less than 15%), but significantly enhances the wall’s deformation capacity and energy dissipation, with increases of 45.57%~72.61% and 79.51%~128.32%, respectively, while also effectively delaying the stiffness degradation process. These effects are particularly strong in slightly damaged or lower-level walls. With its low cost and significant improvement in collapse resistance, this technique has broad application potential in rural dwellings.</p>