A molecular ligand separation method based on multivariate metal-organic frameworks (MOF) is developed to precisely regulate CuSn alloy for tuning the selectivity of HCOOH and CO in CO2 reduction. With this method, the agglomeration and heterogeneous nucleations of metals are effectively inhibited during the in situ electrochemical transformation of CuSn-MOFs into highly pure CuSn alloy. The low Sn content favors CO production, while the high Sn concentration facilitates HCOOH formation.