Passive displacement cooling (PDC) system, eliminating the need for fans for air distribution, has been attracting more attention in green buildings in recent years due to its good thermal performance, IAQ, acoustic comfort, and energy-saving potential. PDC coil and cool air supply duct are the main components of the system. The air supply duct employed in this system takes up valuable floor area, which prevents the development of such a system. In this study, a numerical study of the supply duct length in a PDC room has been conducted, and the performance has been compared under different air supply lengths. The induced air mass flow and heat transfer performance under natural convection have been investigated. The results show that half of the room-height supply duct can perform similarly to the full-height supply duct, resulting in gross floor area savings. The results provide guidance for real PDC system applications in green buildings.