In reinforced concrete (RC) frames, horizontal progressive collapse (H-PC) may be triggered due to insufficient lateral stiffness of the structural systems. Current research studies have been mainly focused on vertical progressive collapse (V-PC) rather than H-PC. To analyze the effect of lateral stiffness and capacity on the behavior of progressive collapse (PC) of RC frames, a static collapse test of two asymmetric one-story beam-column substructure specimens with flange slabs was conducted. The two specimens S1 and S4 were subjected to an interior column removal scenario, in which S1 has one edge column and S4 has an edge and a penultimate column. The test results show that under the compressive arch action (CAA), S1 developed a smaller peak resistance and a larger vertical displacement as compared to S4 due to insufficient horizontal restraint of the columns. In addition, the contribution of the CAA within the beams and slabs in S1 was significantly reduced. Under the catenary action (CA), the collapse resistance of S4 developed rapidly and eventually exceeded the peak resistance of CAA; while the resistance of S1 remained almost constant, caused by the continuously weaken horizontal constraint provided by the edge column. Additionally, the final failure mode for both specimens was found to be in form of the compression-bending failure of the edge and penultimate columns.