Abstract: Static model tests are conducted on three simply supported steel-concrete composite beams under negative moment and on three continuous two-span composite beams. The study focuses on the cracking mechanism and law of crack development, and test results show that transverse reinforcements space generate certain influence on cracking space. According to the test results as well as test data by others, the calculated formula is suggested with the consideration of the transverse reinforcements space and force ratio as two main parameters. At the same time, the formula of uneven coefficient of reinforcement strain in current code is modified, and the calculation formula of crack width for composite beam is suggested. The calculated crack widths are in good agreement with the test results. The results in this paper can provide reference for calculating crack width of composite beams.