In this paper, we construct a Stackelberg-Cournot tripartite game model and discuss the impact of tariff policy on the privatization of the state-owned enterprise, environmental tax, pollutant emission and social welfare in an open economic system. We find that with the increase in tariff, the proportion of privatization of the state-owned enterprise increases, environmental tax falls, and environmental pollution alleviates. The relationship between social welfare and tariff is inverted U-shaped. A closed trade environment is most conducive to environmental protection, but the environmental tax at this time is very detrimental to social welfare. When social welfare is optimal, environmental damage is not the smallest.