To research the external counterfort reinforcement technology for the existing embankment balance weight retaining wall, Geostudio and FLAC3D software were applied to establish the retaining wall slope model of the real project based on the orthogonal experimental design principle and the single factor sensitivity analysis method.The numerical simulation of the reinforced retaining wall structure with the external concrete counterfort reinforced by steel tube of waste guardrail post was carried out.The variation laws were analyzed for the maximum shear stress and maximum tensile stress of the retaining wall body as well as the safety coefficient of slope stability with change in the design parameters of the counterfort reinforcement structure.The results showed that the maximum shear stress and maximum tensile stress of the existing retaining wall exceed the design requirements of mortar rubble wall body strength under the working condition of extreme rainfall, which is the main reason for the retaining wall's vertical cracking and local bulging. After counterfort reinforcement, the maximum shear stress and maximum tensile stress of the retaining wall were significantly reduced, and the safety factor of slope stability was improved considerably. After optimizing the design parameters such as the spacing, thickness, and height of the counterfort as well as the length and spacing of the steel tube, all the checking indexes meet the design requirements.This could be used as a reference for the reinforcement project of the gravity retaining wall.