Urbanization,especially urban land expansion,has a profound influence on the urban thermal environment.Cities in North-east China face remarkably uneven development and environmental issues,and thus it is necessary to strengthen the diagnosis of thermal environmental pressure brought by urbanization.In this study,multi remote sensing imageries and statistical approaches,in-volving piecewise linear regression(PLR),were used to explore urban expansion and its effects on the thermal environment of Changchun City in Jilin Province,China.Results show that Changchun experienced rapid urban expansion from 2000 to 2020,with urb-an built-up areas increasing from 171.77 to 525.14 km2.The area of the city\'s urban heat island(UHI)increased dramatically,during both day and night.Using PLR,a positive linear correlation of built-up density with land surface temperature(LST)was detected,with critical breakpoints of 70％-80％during the daytime and 40％-50％at nighttime.Above the thresholds,the magnitude of LST in re-sponse to built-up density significantly increased with intensifying urbanization,especially for nighttime LST.An analysis of the relat-ive frequency distributions(RFDs)of LST reveals that rapid urbanization resulted in a significant increase of mean LST in newly urban-ized areas,but had weak effects on daytime LST change in existing urban area.Urban expansion also contributed to a constant decrease of spatial heterogeneity of LST in existing urban area,especially at daytime.However,in newly urbanized areas,the spatial heterogen-eity of LST was decreased during the daytime but increased at nighttime due to urbanization.