Diabetes is a global public health problem, and the impact of air pollutants on type 2 diabetes mellitus (T2DM) has attracted people's attention. This study aimed to assess the association of short-term exposure to six criteria air pollutants with T2DM outpatient visits in Lanzhou, China. We collected data on daily outpatient visits for T2DM, daily meteorological data and hourly concentrations of air pollutants in Lanzhou from 2013 to 2019. An over-dispersed passion generalized addictive model combined with a distributed lag non-linear model was applied to estimate the associations and stratified analyses were performed by gender, age, and season. The models were fitted with different lag structures, including single lag days from the current to the previous seven days (lag0 to lag7) and moving average concentrations over seven lag days (lag01 to lag07). A positive association between multiple air pollutants, especially PM_2.5, NO₂, O₃8h and CO and hospital outpatient visits for T2DM was observed. The largest association between T2DM outpatient visits and PM_2.5 was observed at lag06 (RR 1.013, 95% CI: 1.001, 1.027), NO₂ at lag03 (RR 1.034, 95% CI: 1.018, 1.050), O₃8h at lag05 (RR 1.012, 95% CI: 1.001, 1.023) for an increase of 10 μg m⁻³ and CO at lag03 (RR 1.084, 95% CI: 1.029, 1.142) for an increase of 1 mg m⁻³ in the concentrations. In addition, people aged <65 and males are more susceptible, and air pollutants have a greater impact on the cold season. This study showed that although the air pollution in Lanzhou was improved, there was still a statistical correlation between air pollution exposure and T2DM outpatient visits. Therefore, the local government still needs to strengthen the control of air pollution and enhance the protection awareness of the diabetic population through education and publicity.