To enhance post-punching strengths and deformation capacities of flat plate-column joints for mitigating progressive collapse, two strengthening methods including stirrups and embedded ring beams within the joint regions were proposed. An experimental test program on six flat plate-column joint specimens with in-plane constraints was conducted under the modes of upward punching shear failure (UPS mode) and downward punching shear failure (DPS mode). The specimens included two conventional specimens without strengthening (UPS/DPS), two specimens with stirrups (two-legged closed ties, UPS-S/DPS-S) and two specimens with ring beams (UPS-R/DPS-R). The test results showed that, in comparison to UPS/DPS, the average post-punching displacements in UPS-S/DPS-S and UPS-R/DPS-R were increased by 25% and 32%, respectively, along with 64% and 50% enhancement in the corresponding post-punching shear strengths. Numerical simulations were also performed to analyze the effects of different stirrup and ring beam configurations on the post-punching strengths and deformation capacities. Numerical results suggested that double stirrups (four-legged closed ties) installed surrounding the column stub could lessen joint damage and further improve punching shear and post-punching strengths by 15% and 9%, respectively, comparing to UPS-S/DPS-S. For ring beam specimens, under DPS mode, the first punching shear failure inside the ring beams can be prevented by their inner edges directly connected to the column. The post-punching deformations and strengths of this specimen were increased by at least 10%, comparing to DPS-R. Numerical simulation of UPS-R further showed that the strengthening effect also depends on the constraints provided by the concrete.