Abstract: Using the energy-variation principle and considering the influence of the concrete cracking and the slip effect between the steel girder and concrete slab, the analytical stress solutions of the beams under the concentrated load are obtained based on the concept of effective span length, and the effective flange width at negative moment zone of composite beams is also developed. All the numerical results are compared with the test results, and the influence of concrete cracking on the shear lag effect and the effective flange width are investigated. The study suggests that the concrete cracking leads to a smaller shear lag coefficient and a larger effective flange width, while the length of the cracking zone has not much influence on the shear lag coefficient and the effective flange width. It is also found that the effective flange width can be calculated by using a completed cracked beam with a uniform cross-section in the range of effective span length corresponding to the negative moment zone. The actual position of inflection point should be carefully considered for the continuous composite beam. Using the fixed values of the effective span length given in the design codes may involves significant errors in some cases.