This paper examines a causal relationship between foreign direct investment (FDI) and firms' pollution intensity by exploiting the policy of China's FDI access relaxation in 2002. The result shows that FDI leads to a significant reduction in firms' pollution intensity. The mechanism tests find that FDI reduces pollution intensity by increasing firms' productivity, pollution management abilities, and the output of lightly polluting firms. The effect primarily acts on firms in lightly polluting industries and firms in the eastern region. The findings support the pollution halo hypothesis and provide implications for developing countries like China by evaluating the effectiveness of policies to attract FDI.