SummaryDue to recent advances in computational power, high efficiency and high precision semi‐rigid calculation methods of steel connections are necessary. In this paper, four connection experiments were designed to study the mechanical behavior of the top and seat angle connections. In the experiment, a monotonic load was applied at the end of the beam to study its moment–rotation characteristics. In addition, this paper proposed a second derivative method to determine the yield point, the starting point of the plastic strengthening stage, and the initial rotational stiffness of the connections according to the moment–rotation curve. Loading test experiments (EX) results and finite element analysis (FEA) results showed that the compression side angle stiffener has little effect on the initial rotational stiffness. Nevertheless, it can delay the top and seat angle connection in entering the elastoplastic stage and the starting point of the plastic strengthening stage. The results also showed that the tension side angle stiffener could significantly increase the initial rotational stiffness, but makes the angle connection enter the yield stage earlier. When the connection's top and seat angles have no stiffener, the elastoplastic deformation range on the curve is smaller. Finally, based on the theory of Plates and Shells, this paper proposed the calculation method for the deformation stiffness of the key components and the initial rotational stiffness of the connection. Results from the theoretical calculation method showed high calculation accuracy and high calculation efficiency, as compared with the EX and the FEA results.