Falling debris impact from damaged upper structures is a key cause of building progressive collapse, yet relevant research lags behind that on column removal scenarios. This study uses ANSYS/LS-DYNA 16.0 to simulate the dynamic responses of steel-framed subassemblies with five typical beam–column connections under debris impact, with the finite element model validated by drop hammer tests and showing good agreement with the experimental results. Parametric analyses are conducted to explore the effects of the impact velocity, impactor mass, impact energy, and horizontal restraint on structural responses. The results show that under the same impact energy, the velocity and mass significantly affect the maximum impact force but barely the stable-stage force and maximum displacement; horizontal restraint exerts negligible effects at a low impact energy while a single horizontal restraint markedly impairs impact resistance at high energy. These findings are clarified via energy conservation, momentum theorem, and anti-collapse mechanisms. The study’s originality lies in systematically investigating the dynamic responses of the five subassemblies, deriving quantitative relationships between the impact parameters and impact force, duration, and horizontal restraint. It provides theoretical and technical support for anti-progressive collapse building design.