In order to study the interface mechanical behavior of ultra-high performance concrete (UHPC) filled square steel tube, 18 UHPC-filled steel tube specimens were designed for static push-out test, and the width-to-thickness ratio, height-to-width ratio, and the compressive strength of UHPC were considered as main parameters. The failure process and patterns, load-slip curves and bond strength of specimens, and the longitudinal strain of steel tubes were analyzed. The results show that the specimens are intact after the test and there is no visible local buckling on steel tubes, while some damage is formed around the edge concrete at the loading end. The load-slip curves are essentially the same at the loaded and free ends, and the curves are divided into two categories: weakened with a clear peak point and strengthened without a clear peak point. The bond strength decreases with the increase of the width-to-thickness ratio and height-to-width ratio. When the width-to-thickness ratios are large, increasing the strength of UHPC can significantly improve the bond strength. The longitudinal strain at the loading end of the steel tube is slightly larger than the value at the free end, which exhibits a negative exponent distribution along the height. Based on the composition of bond strength, the expressions of interface friction stress and mechanical interaction stress are determined by ignoring chemical adhesive force. The calculation model of bond strength for UHPC-filled square steel tube specimens under two curing conditions are respectively proposed, and the theoretical results agree well with the experimental data.