Flat plate-column joints with in-plane constraints exhibit a suspension mechanism under large deformations after punching shear failure, which significantly affects the progressive collapse behavior of the entire flat plate structural systems. However, in-plane constraints were ignored in the existing tests by which the suspension mechanism could not be exhibited and consequently the post-punching resistance was significantly underestimated. To fill this gap, a static pushdown test was conducted on eight flat plate-column joints with in-plane constraints until their post-punching resistances being completely lost. The influences of the punching directions and the structural parameters including slab thickness, reinforcement amounts and arrangement of flexural reinforcement on the post-punching failure mechanism were analyzed. An analytical method for calculating the post-punching strength was proposed and validated by the test results. The contributions of each layer of reinforcement to the post-punching resistance were also quantified.