The first cementing interface refers to the interface between the cement sheath and the casing after the cement injection is completed. The bonding strength of the first cementing interface is mainly generated by the interaction between the outer surface of the casing and the cement sheath. This article explores the influence of cement strength on the first interface of cementing by changing the water-cement ratio of the cement slurry system, adding ultra-fine materials, and changing the type of fluid loss control additive. Cement slurry types were selected in the form of progressive experiments, including pure cement slurry systems [L1, L2] under different water-cement ratios, cement slurry systems [L3] based on pure cement systems mixed with nano-silica, and the addition of different types of fluid loss control additives [S2-S4] and cement slurry system [S5] with fluid loss control additives and latex added together. Through the self-developed cementing strength evaluation mold and the microscopic characterization of the cementing interface, the influencing factors of the cementing strength of the first interface of the cement sheath were explored. The results show that increasing the water-cement ratio or adding nano-silica can promote the formation of hydrated calcium silicate [C-S-H], with lower early strength but higher later strength; adding a fluid loss control additive can increase the content of calcium hydroxide crystals [CH], the close packing of CH and without hydration reacting cement particles can provide the first interface cementing strength; the reaction of the latex particles themselves to form a three-dimensional network structure can improve the cementing strength, but it is not suitable for high-temperature environments. This research is of great significance to the summary of cementing-interface cementing laws and the cementing construction process.