Abstract: The existence of concrete slab has significant influence on the local mechanical characteristic of the beam-to-column joint. In the design of beam-to-column joints, if the composite effects of steel beam and concrete slab are considered particularly as safety stock, the counteractive result of "strong column and weak beam" or "strong beam and weak column" may be produced; therefore, ignoring the influence of concrete slab affect the bearing capacity and stiffness of the joints. Based on the results of a full-scale cyclic test conducted on T-shaped beam-to-column joints, a detailed nonlinear finite element model was proposed. To completely understand the working mechanism of composite joints and further improve the experimental research, the nonlinear material properties and the shear effect of the connectors between concrete slab and steel beam were considered in the model. The results from the model agree well with the test results. Moreover, an extensive parametric study was performed to examine the seismic behavior of the joints. Parameters, such as the size effect, axial-load ratio, slab thickness, concrete compressive strength, and diameter-to-thickness ratio of the column, were investigated. The results show that the size and axial-load ratio have insignificant effect on the flexural capacity and stiffness of beam end, whereas the slab thickness, concrete compressive strength and diameter-to-thickness ratio of the column have significant effect on it. Furthermore, a calculation formula of flexural capacity was developed to estimate the flexural capacity of the beamto-column joints, considering the composite effects of the slab. The comparison between the calculation, experiment, and simulation results indicates that the proposed formula can reasonably predict the flexural strength of beam-to-column joints with concrete slab.