Ultra-high voltage (UHV) transmission projects provide an effective way to alleviate the reverse distribution of energy in China, but do they reduce regional carbon emissions? This paper takes UHV transmission projects as a quasi-natural experiment and adopts a time-varying difference-in-differences (DID) method to test the effect of trans-regional electricity transmission on carbon emission performance and its transmission path. The empirical results show that (1) UHV transmission projects have significantly reduced the amount and intensity of regional carbon emissions, and this conclusion is still valid after a series of robustness tests. (2) UHV transmission projects can reduce regional carbon emissions by promoting energy substitution, power generation capacity, industrial agglomeration, and output efficiency. (3) The carbon reduction effect of UHV transmission projects is more significant in regions with high energy dependence and regions with clean power generation at power output. The findings of this paper not only enrich the literature on infrastructure investment and carbon emission reduction but also provide an empirical reference for the government to orderly promote the new infrastructure investment direction.