To address the problem that cut-fill roadbed slopes in mountainous areas are prone to instability and sliding failure under the influence of rainfall and cracks, local reinforcement of the slopes was carried out using geosynthetic reinforced soil technology. Based on the upper bound theorem of limit analysis, a calculation formula for the stability of locally reinforced cut-fill slopes considering pore water pressure was established. Through parameter analysis, the influence laws of local reinforcement on slope stability and that of weakening coefficient on failure modes were revealed. The results show that the required reinforcement force for the stability of cut-fill slopes increases with the increase of the fill slope and pore water pressure, while it decreases with the increase of reinforcement depth and the original slope. Increasing the local reinforcement depth significantly improves slope stability, but the possibility of translational failure surface needs to be considered. When the pore water pressure is high, rotational failure should be considered.