In order to study the influence of residual mud during construction process and steel pipe with different radius-thickness ratio on the flexural behavior of concrete filled steel tube, six steel tubular composite pile specimens were prepared by simulating underwater pouring of concrete for bending experiment and cross-section measurement. Through the test, M-um，M-Φ curves，flexural resistance and stiffness of these specimens were obtained. The test results show that the existence of mud weakens the stiffness and flexural resistance of concrete-filled steel tube by approximately 5%~25% and 5%~10%, respectively. The smaller the radius-thickness ratio is, the more obvious the influence of mud is. The steel tube and concrete no longer meet the section assumption under the influence of the mud. The flexural capacity calculated by the unified theory agrees well with the test value, but with the increase of mud thickness, it gradually becomes larger than the test values, which leads to the reduction of the reserve for safety in design. The coefficient of the stiffness formula is about 0.1~0.4.