In this study, we evaluated the variations of air quality in Lanzhou, a typical city in Northwestern China impacted by the COVID-19 lockdown. The mass concentration and chemical composition of non-refractory submicron particulate matter (NR-PM) were determined by a high-resolution aerosol mass spectrometry from January to March 2020. The concentration of NR-PM dropped by 40% from pre- to during control period. The five aerosol components (sulfate, nitrate, ammonium, chloride, and organic aerosol (OA)) were all decreased during control period with the largest from secondary inorganic species (70% of the total reduction), whereas the OA sources did not vary synchronously. OA from coal and biomass burning remained stably from pre- to during control period, while traffic and cooking related emissions were reduced by 30% and 50%, respectively. The production rates for secondary inorganic and organic aerosols were also evaluated and represented a decreased trend from pre- to during control periods.