Abstract: The local cracking problem of steel-UHPC (ultra-high-performance concrete) composite structures under negative bending moments is the basis of the application and development. Based on the UHPC tensile bilinear constitutive model and plane cross-section assumption, a section stress method is set up to describe the stress distribution of the steel-UHPC composite slabs and calculate the cracking load under a pure bending condition. The proposed method is verified by 4 specimens of 2 groups and the test results in the literature. The results indicate that the section-stress method can accurately describe the cracking behavior of steel-UHPC composite plates under pure bending and the average ratio of calculated to measured values is 0.95. The UHPC cracking strength is an intrinsic property dependent on the nature of the material itself and is not influenced by the steel-UHPC composite slab. The height of the UHPC strain hardening zone represents the combination effect and reinforcement constraint when visible cracks appear. By reducing the cover thickness or increasing the reinforcement ratio, the UHPC strain hardening height can be increased and the flexural behavior of the steel-UHPC composite slab can be improved.