The Huang-Huai-Hai Plain (HHH) is typical of China’s farming area and was predicted as one of the fastest growing areas of urbanization in the world. Since the turn of the new millennium, construction land and farmland transitions in this region driven by rapid urbanization have resulted in a dramatic loss of farmland, which triggered a serious threat to regional and even national food security. In this paper, the coupling relationships between per capita construction land transition (PCCT) and per capita farmland transition (PCFT) in the HHH and their implications for regional food security are analyzed. During 2000–2015, the farmland decreased by 8.59%, 72.25% of which were occupied by construction land. There are two major coupling types between PCCT and PCFT, one is the double increasing of per capita construction land area (PCCA) and per capita farmland area (PCFA); another is the increasing of PCCA and the decreasing of PCFA. The fluctuant increasing of PCCT and decreasing of PCFT coexisted and presented symmetrical coupling characteristics in space. Physical, location, transportation, and socio-economic factors play significantly different roles in driving PCCT and PCFT. The implications for ensuring food security involve promoting the reclamation and redevelopment of inefficient and unused urban–rural construction land, reducing inefficient occupation of farmland resources, developing appropriate scale management of agriculture, and establishing a better social security system to smoothly settle the floating rural population in the city.