Abstract: In recent years, the concentrations of multiple air pollutants in China have been reduced yet the reduction of surface ozone (O₃) concentration is not satisfactory compared to that of fine particulate matter (PM_2.5). The co-occurrence of PM_2.5 and O₃ pollution has occurred from time to time. To develop emission reduction strategy for PM_2.5 and O₃ on concurrent pollution days, the emission reduction effects of anthropogenic NO_x and volatile organic compounds (VOCs) in Shanghai were simulated using a three-dimensional meteorological-air quality model for the period of April 27—30, 2018, when PM_2.5 and O₃ both exceeded the national standard in Shanghai. Simulation results show that Shanghai was mostly under VOCs-controlled during the modeling period with a peak ratio (PR) of 0.6-1.1 in terms of the maximum daily eight-hour average (MDA8) O₃. A VOCs/NO_x reduction ratio (VNr) of at least 1.9:1 is needed to prevent ozone increase during emission reduction maintain the MDA8 O₃ concentration in Shanghai. For the reduction of PM_2.5 concentration, regional transport, SOA response to precursors and the diurnal variations in NO₃^- production are important factors. Compared to the local emission reduction in Shanghai, both emission reduction advanced in time and jointly in region can further reduce the MDA8 O₃ concentration in Shanghai by 0.6%-3.0%. However, under the regional emission reduction scenario, PM_2.5 concentration in the downwind (i.e. southwestern Jiangsu) can be effectively reduced whereas PM_2.5 concentration slightly increased in Shanghai due to increases of the secondary inorganic components in PM_2.5. Advanced emission reduction (by three days ahead of pollution days) is more effective than regional reduction in terms of reducing PM_2.5 concentration in Shanghai. In order to effectively reduce both MDA8 O₃ and PM_2.5 concentrations, the emission reduction plan should be consider the local emissions of VOCs and NO_x in different regions as well as the meteorological conditions. To effectively achieve reductions inPM_2.5 concentration, attention also needs to be paid to the emission reduction of primary particulate matter and other gaseous precursors (e.g. SO₂, NH₃).