Abstract: STS (Steel Tube Slab) structure was used to construct subway station as a primary support. To investigate the flexural capacity of STS members, eight STS members were fabricated and tested under a concentrated load. The major parameters included connecting bolts, welding of flange plate, continuous bolts, spacing of adjacent steel tubes, flange thickness and concrete compressive strength. The experimental results indicate that the ultimate capacity and stiffness of STS member was increased significantly with the welding of bottom flange. Connecting bolts, increased flange thickness, concrete compressive strength and continuous bolts had a limited effected on the flexural capacity of STS members, but effectively improved the stiffness of STS members. The stiffness and ultimate flexural capacity of STS member can be little enhanced by the upper flange welding or smaller spacing of adjacent steel tubes. A calculating formula was proposed to predict the flexural capacity of the STS members, and good agreement between the tested results and calculation values was obtained.