A fundamental study on C-C coupling, that is the crucial step in the Fischer-Tropsch synthesis (FTS) process to obtain multi-carbon products, is of great importance to tailor catalysts and then guide a more promising pathway. It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process, while the related mechanism is elusive. Herein, the reactions of the CuC$ _3 $H$ ^- $ and CuC$ _3 $$ ^- $ cluster anions with CO have been studied by using mass spectrometry and theoretical calculations. The experimental results showed that the coupling of CO with the C$ _3 $H$ ^- $ moiety of CuC$ _3 $H$ ^- $ can generate the exclusive ion product COC$ _3 $H$ ^- $. The reactivity and selectivity of this reaction of CuC$ _3 $H$ ^- $ with CO are greatly higher than that of the reaction of CuC$ _3 $$ ^- $ with CO, and this H-assisted C-C coupling process was rationalized by theoretical calculations.