As the largest developing country, China faces significant air pollution challenges despite rapid socio-economic progress. This paper focuses on thirty-five cities in the three northeastern provinces of China – Heilongjiang, Jilin, and Liaoning – analyzing air quality trends from January 2015 to March 2024 and providing a forecast for the period from April 2024 to April 2025. We utilize an improved TOPSIS method combined with Set Pair Analysis and a Prophet model for forecasting, enabling comprehensive air quality assessment and trend prediction. The findings show significant improvements in air quality across thirty-two cities, particularly in the provincial capitals, largely attributed to effective pollution control policies. However, spatial and seasonal variations persist, with air quality declining during winter heating seasons, especially in cities heavily reliant on coal. This paper concludes with policy recommendations, emphasizing the need for further energy structure adjustments and strengthened regional pollution control mechanisms to sustain these improvements.