In order to study the influences of yield strength fy and concrete cover thickness h of shear reinforcements on the direct shear behavior of monolithically cast concrete interfaces, push-off tests on 12 Z-shaped specimens were conducted. Based on the experimental phenomena and measured load-slip curves, the mechanism of interface shear transfer was proposed. According to the tested bearing capacity of the specimen, the formulas from ACI, PCI and AASHTO codes for calculating the interface direct shear capacity were analyzed and evaluated. The analysis results show that when fy increases from 400 MPa to 600 MPa or h increases from 20 mm to 40 mm, the direct shear strength and shear stiffness of interface do not change significantly, while the change of h will cause the different interface failure modes. The adhesion and cohesion of concrete contribute greatly to the shear capacity of interface. All calculation formulas from the three codes mentioned above can be applied to guide engineering design, while the ACI and PCI calculation formulas seem to be excessively conservative, and the AASHTO calculation formula has a higher calculation accuracy. The research conclusions of this paper can provide a reference for the configuration of the shear reinforcements in monolithically cast concrete interfaces and the selection of relevant design codes.