Abstract: To study further the ultimate flexural capacity of a steel tube member filled with steel-reinforced high-strength concrete, its failure is divided into several modes according to the different positions of neutral axis. Then, an improved superposition method is introduced in the calculation of the cross-sectional bending capacity of flexural members of the rectangular steel tube beams filled with steel-reinforced high-strength concrete and the formula for typical failure mode is given to calculate the bending capacity in tensile region under conditions that both compressional and tensile region have yielded. The new method overcomes the shortcoming of simple and conventional superposition methods in calculation, which provides higher precision and the formula proposed can offer reference for engineering practice.