A method to connect modular steel frame units(hereinafter referred to “modular units”) was proposed, which is that 2, 4, or 8 modular units can be connected by innovative box connecting component(BCC-1and BCC-2), plates through high strength bolts. Force mechanism of box connections were analyzed, and based on the Chinese code, the design flow and design method of the inovative box connections (hereinafter referred to “box connections”) of modular units were proposed. Taking the 11 layers modular steel frame office as an example, an corner connection and an edge connection were designed, and low cycle reciprocating loading were carried out on the connection using the finite element software Abaqus to verify seismic resistance of modular connections. Results show that:plastic hinge of corner connections and edge connections initialed on web of BCC-1 and BCC-2, which can be quickly replaced after an earthquake to restore the function of connections. Both connections met the requirements of the ANSI/AISC 360-10 specification for semi-rigid connections, and hysteresis curve were full, indicating good seismic resistance. Both ratio of ultimate bending capacity of predicted value and finite element results exceed 90%, indicating that the structure, the idea, and the design method are reasonable.