Modular steel structures as an efficient， environmentally friendly， and sustainable construction method， and the connection between the units will directly affect the overall stability and seismic performance of the structure. However， the common connection methods in modular steel structures， such as welding， bolted connections， and prestressed connections， exhibit several challenges including low construction efficiency， susceptibility to installation errors， and limited working space. In this paper， a novel shear key double-layer grouting connection is proposed， which has good applicability to corner column joints， side column joints and middle column joints of modular steel structures. In order to evaluate the axial mechanical properties of the novel connection， 8 connection specimens were produced for push-out tests， and the effects of shear key spacing， grout strength， connection length and number of grout layers on the failure forms， tensile capacity and strain distribution of the specimens were investigated. Based on the theoretical analysis， the formula for calculating the tensile capacity of the shear key double-layer grouting connection is derived. The effectiveness and accuracy of the formula are verified by comparing with the test results， which can provide a theoretical basis for the engineering application of the novel connection in the modular steel structure.