Abstract: To promote the application of integrated modular steel construction in high-rise buildings and seismic fortification areas, a new type of fully assembled modular steel structure plug-in connection with additional energy dissipation is proposed, T-shaped connector is used instead of traditional welding connection to achieve full assembly of modular units, the relative slip between the T-shaped connector and the friction plate could provide additional energy dissipation. Based on the force and deformation mechanism of the connection, the formula of initial rotation stiffness is derived. The refined numerical models of the semi-assembled and fully assembled modular steel structure plug-in connection are established to analyze their hysteretic curves, energy dissipation capacity, stress and strain distribution. Results indicate that: the fully assembled plug-in connection exhibits stable parallelogram-shaped hysteretic responses, and the theoretical result agrees well with the numerical result, which effectively verifies the accuracy of the stiffness formula. Compared with the semi-assembled plug-in connection, the bearing capacity of the fully assembled plug-in connection is reduced by 26.59%, the maximum energy dissipation coefficient of connection is increased by 70.06%, T-shaped connector could effectively transfer the plastic damage to the presupposed area, protecting the main components and realizing damage controllable, and significantly improving seismic performance of the connection.