Currently, the international economy is experiencing profound and intricate transformations, while economic globalization is encountering unprecedented challenges and witnessing a surge in counter-globalization sentiments. Critics express concerns over the potential detrimental effects of trade liberalization on the welfare of low-income groups, particularly in developing countries. China, as the largest developing country, also holds the status of a major trading nation. Using panel data from 285 prefecture-level cities across China during 2000–2019, this study examines how trade openness affects population health to assess the welfare effects of trade openness, providing new theoretical perspectives and empirical evidence to further promote economic globalization and improve human health and well-being. The results show that trade openness can significantly promote population health. And, a series of robustness tests show that the above conclusion is still valid. Trade openness has a greater positive impact on the health of the population of the eastern and central regions, the coastal cities, and high-trade-openness cities. An analysis of the mechanism reveal that trade openness mainly promotes population health through channels such as labor employment, wage income, public health investment, and personal health investment. However, trade openness can also lead to environmental pollution, which has a significant negative impact on population health. Additionally, through threshold effects analysis, it becomes evident that the relationship between trade openness and population health is nonlinear. This implies that there are critical thresholds related to the economic development stage, investment in environmental governance, and environmental infrastructure construction. Falling below or exceeding these thresholds may lead to different interval effects on the impact of trade openness on population health.