Abstract: Five types of full lightweight ceramsite concrete and ordinary concrete composite specimens with different interfacial agents were produced. Moreover, mechanical tests (splitting tensile, shear and bending tests) and SEM tests were performed to investigate the effect of interfacial agents on the mechanical properties of the interface between full lightweight ceramsite concrete and ordinary concrete. The mechanical test results show that firstly, the applying interfacial agent can effectively improve the structure of the interfacial zone and substantially enhance the mechanical properties of the interface. For interfacial splitting tensile strength and flexural strength, epoxy resin is the optimal interfacial agent and the strength values will be increased by 56.5% and 38.3%, respectively. The polymer mortar has the most significant effect on the improvement of interfacial shear strength, which can be improved by 71.2%. The mechanical properties of the interface coated with cement paste can also meet the requirements of industry standards. As an interfacial agent, cement paste containing silica fume is superior to cement mortar. Secondly, the influence degree of interface agent on mechanical indexes from strong to weak is splitting tensile strength, flexural strength and shear strength. Thirdly, based on the mechanical indexes of full lightweight ceramsite concrete, a formula for calculating the mechanical strength of the interface between old and new concrete considering the influence of interfacial agents was established. Finally, SEM testing provides a good explanation of the conclusions of mechanical performance testing at the micro level. The use of interfacial agent can reduce the microcrack width between new and old concrete. Among them, the effect of epoxy resin interfacial agent and polymer mortar is more obvious. Besides, it can effectively reduce the porosity of the interfacial transition zone, with a decrease of 46.44%-60.81%. The research conclusions have reference value for the interface treatment of concrete structure reinforcement.