This article presents design theory for the strength of the cross-section of steel box concrete composite beam, based on the analysis of its mechanic behaviors, analyzes the influence of width- thickness ratio and confined effect on the strength, and puts forward the principles for rational design of its dimensions. The analysis shows that the bearing capacity of such new composite beam increases compared with that of empty steel box beam, and its mechanic behavior is obviously improved. Experimental study of 3 beams shows that steel box composite beam has good behavior of bending resist and flexibility and high bearing capacity. Such kind of beam has widely application prospect after further theoretical and experimental study.