Abstract: To study the mechanical properties of concrete filled square steel tube columns with spiral reinforcement under eccentric compression, 18 specimens were designed and tested under eccentric compression with the spacing of the spiral reinforcement, diameter-to-width ratio, slenderness ratio, eccentricity ratio, diameter of the longitudinal rebars and thickness of square steel tube as variable parameters. The failure processes and failure modes of the specimens were observed, and the load-displacement curves and the distributions of steel strain were obtained in the test. The influences of various parameters on the mechanical properties of the specimens under eccentric compression were analyzed. The results show that the ductility and bending stiffness of the specimens were effectively improved by the spacing and diameter-to-width ratio of the spiral reinforcement. With the increase of the slenderness ratio, the mechanical properties of the specimens are decreased. With the increase of the eccentricity, the ultimate strength, ductility, and flexural stiffness of the specimens were significantly decreased, while the energy dissipation coefficient was obviously increased. A program compiled by a fiber-based model was used to calculate the ultimate strengths of the specimens under eccentric compression. The calculated values were in good agreement with the test results. Based on this program, an extended parametric analysis was carried out, and the optimal steel distribution mode and some design suggestion were proposed.