Under combined air pollution conditions, the coordinated control of PM_2.5 and O₃ has significant implications for environmental management. To better understand the coordinated control capability of different metropolises, a prime consideration is how to develop a scientific and objective index for quantifying it. In this paper, based on the multifractal detrended cross-correlation analysis (MF-DCCA) method, a novel dynamical index $\eta$ is developed to quantitatively determine the capability of metropolis to control PM_2.5 and O₃ pollution cooperatively. Furthermore, combing with the sliding window method, the high temporal scale resolution of $\eta$ values of four metropolises (Beijing, Shanghai, Guangzhou, and Chengdu) in China is conducted in order to determine the temporal evolution of $\eta$. Then, the monthly variation of $\eta$ values in the four metropolises is investigated. The U-shaped curves in the monthly mean $\eta$ values indicate that PM_2.5-O₃ coordinated control capability of the four metropolises is best in summer. The further discussions about the relations between index $\eta$ and meteorological conditions have been made. Next, long-term trends of $\eta$ series of four metropolises are studied by Mann–Kendall statistical test. Result shows that the trends of the $\eta$ series of four metropolises are similar from 2015 to 2017. After 2017, the $\eta$ series of Beijing, Shanghai and Chengdu show an increasing trend, while that of Guangzhou show a decreasing trend. This suggests that PM_2.5-O₃ coordinated control capability of Guangzhou is gradually strengthened after 2017. This may be caused by the combined effect of targeted strategies and special climatic conditions in Guangzhou. The $\eta$ index could directly quantify the two-pollutant coordinated control capability of different metropolises based on field observation, rather than rely on any subjective factor. It can provide guidance for policy-making of pollution emission reduction, so as to control the combined air pollution.