Abstract: This paper presents a high fill slope solution for the steep and karst-developed terrain at the southern end, west side of Wulong Airport in Chongqing, where a combination of high embankment and ultra-high counterweight retaining wall was adopted. The foundation of the gravity retaining wall includes three karst formations, covering more than 45% of the total area. These karst areas are fully filled and have a maximum depth exceeding 30 m, making them typical examples of special and complex foundations. To address challenges such as strong non-uniformity, low bearing capacity of the karst foundation, and instability of slopes and retaining walls in karstic foundations, a solution involving the excavation of karst infill to a certain depth and backfilling with concrete was adopted. Through theoretical calculations, this paper comprehensively analyzes the failure modes, stability, stress, and deformation of the high slope and retaining wall with varying concrete replacement depths, ultimately determining a suitable replacement depth. The research results show that the adoption of a certain depth of replacement can effectively improve the non-uniformity of the karst foundation, reduce stress concentration effect on the retaining wall, decrease the deformation of the retaining wall and high embankment, and significantly enhance the stability of the retaining wall and slope. Field monitoring indicates that the horizontal and vertical displacements of the high retaining wall and high slope after construction are both less than 4mm, with deformation curves converging, demonstrating good stability of the slope and retaining wall. The research findings have significant reference value for the planning, design, and construction of high-fill slope projects in complex mountainous areas.