Re-dispersible latex powder is the key material for the preparation of steel structure interface mortar. In this paper, the influence of the content of re-dispersible latex powder on the 28 d compressive strength, flexural strength and tensile bond strength of interfacial mortar was researched. At the same time, combined with scanning electron microscopy, X-ray spectroscopy and other microscopic analysis methods, the bonding mechanism of re-dispersible latex powder to enhance the interfacial mortar was analyzed. The results show that the suitable content of re-dispersible latex powder is 15% (mass fraction， same below) in the preparation of steel structure interface mortar. Under the appropriate dosage, the tensile bond strength of the interface mortar reaches 1.12 MPa, and the compressive-flexural strength ratio is 2.3. The addition of latex powder can effectively improve the flexibility of the mortar. The addition of latex powder changes the pore structure of the mortar, and the porosity becomes larger. The hydration products in interfacial transition zone are mainly polymer film encapsulated hydration products C-S-H gel and Ca(OH)2.