Diaphragm through connection is a new kind of connection between steel beam and concrete-filled steel tubular column. Quasi-static test and numerical simulation had been carried out to investigate the behaviors of diaphragm through connection used in Zhonghai Square. The results show that, the bearing capacity, rigid and ductility of the diaphragm through connection are excellent, and the seismic principle, i.e., ‘strong column but weak beam’ and ‘strong connection but weak component’, is reached. The numerical model of diaphragm through connection was provided, and the results drawn from the model agreed with the test results. Influences of diaphragm dimension on the behaviors of the diaphragm through connection with praposed the numerical model had been investigated, and the design suggestion was provided based on the analysis.