The spatiotemporal evolution of cropland exerts substantial impacts on regional thermal environments. However, the biophysical pathways through which cropland dynamics modulate land surface temperature (LST) remain insufficiently quantified. In this study, we integrated multi-source remote sensing and reanalysis datasets from 2000 to 2020 to systematically investigate the biophysical mechanisms linking cropland evolution to LST variations across China. The results revealed a pronounced spatial redistribution of cropland from the southeast toward the northwest, characterized by continuous expansion in northern and western provinces and persistent loss in southeastern coastal regions. Although the nationwide mean temperature difference between cropland and adjacent natural ecosystems was nearly zero, marked spatial heterogeneity was evident: cropland in northeastern China exhibited a distinct cooling effect, whereas that in arid northwestern regions induced significant surface warming. Among the biophysical drivers, ground heat flux (27%), surface emissivity (26%), and albedo (22%) were identified as the primary contributors to LST variability. Overall, cropland evolution plays a crucial role in regulating surface energy fluxes and regional thermal regimes over long timescales. Cropland expansion in arid northwestern China amplified surface warming, while in northeastern regions, cropland generally exerted a persistent cooling influence. However, this cooling effect may be temporarily offset during periods of rapid expansion due to short-term warming induced by surface drying, soil exposure, and shifts in energy partitioning. These findings enhance the mechanistic understanding of land-use change-climate feedbacks and provide scientific support for climate-resilient agricultural development and sustainable land management.