Abstract: In underground structures, middle partition walls with thin cross-sections and low stiffness often constitute seismic weak links. To address this issue, a prefabricated middle partition wall system connected by Ultra-High Performance Concrete (UHPC) grouted sleeves was proposed, and its seismic performance was systematically evaluated through 1:1 full-scale quasi-static tests. The results showed that the UHPC grouted sleeve connections possessed reliable connection strength and force transfer capacity; the failure mode of the specimens exhibited typical bidirectional crushing at the wall base, accompanied by extensive concrete spalling, yielding of longitudinal reinforcement, and concentrated formation of plastic hinges; concrete strain monitoring data confirmed that the wall base region experienced the maximum bending moment, representing the critical control location for seismic design; UHPC grouted sleeves could effectively enhance the integrity and ductility of prefabricated middle partition walls.