China's COVID-19 lockdown closed many of its air pollution sources and created an unprecedented clean-air scenario. However, air pollution remained significant, arousing questions about its causes and reawakening debate over the country's ultralow emission (ULE) policy. Here, we analyze the characteristics and causes of China's air pollution during this period to assess ULE's effectiveness. We find that the concentrations of fine particles (PM_2.5, PM₁₀), NO₂, and CO in Wuhan decreased significantly during the lockdown. However, O₃ and SO₂ concentrations increased, though to different extents. The ULE policy promoted emission reductions, but emissions from coal-fired industries (e.g., thermal power and steel) remained the fundamental source of air pollution and appeared to have been underestimated in previous researches. In contrast, emissions from construction and transportation and the ULE's effectiveness may have been overestimated. The characteristics of air pollution in southern and northern cities support these conclusions. Therefore, it is necessary to strengthen the adjustment of China's energy and industrial structures, and identify more suitable mitigation measures for each region's characteristics. It is also necessary to strengthen the synergistic governance of pollutants and promote emission reduction technologies.