Although Sn-based catalysts have recently achieved considerable improvement in selective electro-catalyzing CO₂ into HCOOH, the role of various valence Sn species is not fully understood due to the complexity and uncertainty of their evolution during the reaction process. Here, inspired by the theoretical simulations that the concomitant multivalent Sn (Sn⁰, Sn^II and Sn^IV) can significantly motivate the intrinsic activity of Sn-based catalyst, the Sn/SnO/SnO₂ nanosheets were proposed to experimentally verify the synergistic effect of multivalent Sn species on the CO₂-into-HCOOH conversion. During CO₂ reduction reaction, the Sn/SnO/SnO₂ nanosheets, which are prepared by the sequential hydrothermal reaction, calcined crystallization and low-temperature H₂ treatment, exhibit a high FEHCOOH of 89.6% at –0.9 VRHE as well as a large cathodic current density. Systematic experimental and theoretical results corroborate that multivalent Sn species synergistically energize the CO₂ activation, the HCOO* adsorption, and the electron transfer, which make Sn/SnO/SnO₂ favour the conversion from CO₂ into HCOOH in both thermodynamics and kinetics. This proof-of-concept study establishes a relationship between the enhanced performance and the multivalent Sn species, and also provides a practicable and scalable avenue for rational engineering high-powered electrocatalysts.