Abstract: Taking the cold-formed thin-walled U-section steel-concrete composite beams as material, theory analysis have been conducted on the composite beams. Expressions for elasitic strength in consideration of slip effect were given. The bending moment of eight simply supported composite beams were calculated, and the comparison between theory and test results showed that the calculated values of elasitic monent was good consistent with the test results. It is shown that the slip effect on the elastic bending strength can not be ingored.