The relationship between fine particulate matter (PM_2.5) and respiratory disease outcomes among children aged 0 to 14 years in Lanzhou, China, was evaluated. We utilized a generalized additive model linked by a quasi-Poisson distribution to examine the associations between PM_2.5 and paediatric respiratory outpatient visits for time lags of 0 up to 7 days, and stratified by gender, age, and season. Cases of respiratory disease in children were collected from 3 large hospitals for the years 2014–2017 and then linked with air pollutant concentrations from 4 air quality monitoring stations by date. We observed positive and significant associations between PM_2.5 and respiratory disease from the lag to lag 7, and from lag01 to lag07, with ER reaching the maximum value at lag07. For each 10 μg/m³ increase in PM_2.5 (lag07), the associated increment in respiratory diseases was 2.83% (95% CI 1.80%–3.86%). Males were more sensitive to the adverse effects, and the association was more significant in spring (from March to May) and winter (from December to the next February). Overall, the child group (age 3–6 years) demonstrated a higher risk of respiratory disease after PM_2.5 exposure. The associations between ambient PM_2.5 and respiratory hospital outpatients among young children became partially attenuated after the adjustment for gaseous pollutants in subgroups. The exposure-response curves were positive and generally nonlinear but flatted at concentrations over 60 μg/m³. This research found a significant association between ambient PM_2.5 levels and hospital outpatient visits in child with respiratory diseases in Lanzhou, China.