Background Research on the relationship between ambient temperature and preterm birth has received increasing attention, but the conclusions of the previous literature are inconsistent. Objective To explore the impact of environmental temperature exposure in Ningbo on premature delivery of pregnant women. Methods The birth information, preterm birth data, and age of pregnant women from January 2016 to September 2020 were collected by the electronic medical record system of Ningbo Women’s and Children’s Hospital. Meteorological data for the same period were obtained through Ningbo Meteorological Bureau, including daily average temperature, daily average relative humidity, and daily average air pressure. Daily concentrations of SO₂, NO₂, and PM₁₀ were derived through the air quality real-time release system on the website of Ningbo Environmental Protection Bureau. A distributed lag nonlinear model was used to analyze the impact of environmental temperature on preterm birth by stratifying pregnant women’s age and birth delivery mode. Results The incidence rate of preterm birth in Ningbo from 2016 to 2020 was 5.91%. The exposure-response curve between environmental temperature and preterm birth presented a “U” shape. Taking 22.5 ℃ as a reference, the cumulative effect of 31 ℃ (the 95th percentile) and 32 ℃ (the 99th percentile) over a 21-day lag on preterm delivery was statistically significant, and the related RR (95%CI) values were 1.67 (1.05-2.65) and 1.85 (1.09-3.14) respectively. The results of stratified analysis showed that among pregnant women ≥30 years old, the 21-day cumulative effects of 31 ℃ and 32 ℃ on preterm delivery were statistically significant, and the related RR (95%CI) values were 2.09 (1.08-4.05) and 2.36 (1.11-5.03) respectively; among pregnant women with natural delivery, the 21-day cumulative effect of 32 ℃ on preterm delivery was statistically significant, and the RR (95%CI) was 1.95 (1.02-3.74). Conclusion Exposure of pregnant women to high temperature during pregnancy could increase the risk of preterm birth, and there is a delayed cumulative effect.