Developing efficient electrocatalysts for CO2 reduction to syngas with tunable H2/CO ratios and high total faradaic efficiency is challenging. Herein, we report an effective catalyst composed of in situ reconstructed AgZn3 nanoparticles and Zn nanoplates for syngas synthesis, showing nearly 100% Faraday efficiency to syngas with a tunable H2/CO ratio from 2 : 1 to 1 : 2. Moreover, the in situ electrochemical measurements coupled with theoretical calculations disclose that the Zn site in AgZn3 nanoparticles and the hollow site between Ag and Zn in AgZn3 are the possible active sites for CO and H2 generation, respectively. This work has guiding significance for designing dual site catalysts for CO2 electroreduction to tunable syngas.