To improve anti-collapse behavior of an existing masonry structure under earthquake action, adding external ring beams and columns is an effective way to enhance the integrity of the structure. Considering the minimum impact on the facade and the useful space of an existing building, a new thin beam-column system was proposed. In order to verify the efficiency of the strengthening method, as one of the series projects, this paper focuses on the cooperative work behavior of an existing masonry wall and an externally added concrete thin column under horizontal cyclic loads. Six 1/2 scaled specimens of existing masonry corner wall were built. Among them, three specimens were strengthened with externally added thin concrete columns. Then quasi-static tests were conducted to study the seismic behavior of the walls under different axial loads and cyclic horizontal loads from different directions. Test results showed that the externally added thin concrete column could work together well with an existing masonry corner wall by using the construction measures proposed in this paper, and the thin concrete column could be used in the thin beam-column system to enhance the integrity of an existing masonry building.