U-section steel-encased concrete composite beams is the new type composite beam.This kind of composite beam takes thick steel plate as bottom plate and thinner cold-formed thin-walled steel as web which are connected through a u-shaped section by welding, and then concrete are poured in the u-shaped. In this paper an experimental study was conducted on six full size simple supported U-section steel-encased concrete composite beams.The relationships of the load-deflection curves of the composite beams were obtained. From the experimental results,the variation laws of deflection are analysed,and three defletion formulas are analysed and compared. Based on actual code. A theoretical formula is proposed for defletion. The results of theoretical formula are proved to be in good agreement with the experimental results.