The construction land expansion (CLE) in China, based on the occupation of cultivated land, has resulted in a notable alteration of the landscape of cultivated land, leading to an increase in cultivated land fragmentation (CLF) in China. However, previous studies have devoted minimal attention to CLF due to CLE, thereby constraining the coordination of CLE and cultivated land conservation. Accordingly, the study examined the impact mechanism of CLE on CLF utilizing land use/cover datasets and the geographically weighted regression model from 2000 to 2020 in China. The findings indicate that a considerable amount of cultivated land was converted into construction land. The extent of CLF increased from 0.352 in 2000 to 0.383 in 2020, with the majority of this expansion occurring in the urban-rural fringe. CLE, in conjunction with the expansion of construction land in terms of both area and shape, has evolved from a phenomenon confined to individual cities to one that is characterized by regional expansion. The impact of CLE on CLF was found to be spatially heterogeneous over the study period, with an overall weakening trend. In highly urbanized areas, CLE typically promoted CLF, in contrast to low urbanized areas. The area growth and form complexity of the construction land caused by CLE promoted CLF over most regions. Different levels of land urbanization were identified as the primary contributor to the above results. The study findings support decision-making on food production improvement and construction land control.