Castellated composite beam can decrease the floor height of buildings． In order to investigate the bending capacity of castellated composite beam under negative moment，two cantilever beam specimens were designed and tested by applying static loads． The influences of the expansion ratio and location of first web opening on beam mechanical behavior were studied． The results show that bending failures of the specimens occur near the beam support section or the first web opening section for the specimens with the expansion ratio no larger than 1. 3． When the expansion ratio of specimens is maintained unchanged，the location of the first web opening mainly affects on the failure mode of the beam． For both the beam support section and the first web opening section being the strength control section，the expansion ratio of specimens have greatly effect on the strength and stiffness of the beam． Considering both the failure modes and the flexural capacity of two control sections，the method calculating the nominal bending capacity of the castellated composite beam near the support section were presented，and the calculated values fit well with the experimental ones．