A heterogeneous phase combined flooding system composed of polymers, surfactants, and branched-preformed particle gel (B-PPG) has been successfully applied to enhance oil recovery in mature reservoirs. However, the influence of elastic modulus on the enhanced oil recovery of B-PPG is unclear. Thus, based on the sand-pack flooding experiments and visual flooding experiments, the enhanced oil recovery ability of B-PPG with different elastic modulus and similar particle size was investigated under different injection modes. Results show that under the single-slug injection mode, the higher the elastic modulus, the higher the incremental oil recovery, and the better the ability to enhance oil recovery. With the increase in the elastic modulus from 0.7 Pa to 42.2 Pa, the incremental oil recovery increased from 12.7% to 32.6%. Under the multi-slug alternating injection mode, the incremental oil recovery of injecting low elastic modulus B-PPG slug followed by high elastic modulus B-PPG slug was 2.8% higher than that of injecting high elastic modulus B-PPG slug followed by low elastic modulus B-PPG slug. The incremental oil recovery of B-PPG under the multi-slug alternating injection mode was higher than that under the single-slug injection mode. At the microscopic level, the type of remaining oil was mainly clustered after water flooding. With the increase in the elastic modulus, the ratio of the clustered remaining oil decreased and the ratio of the multi-porous, columnar, and droplet remaining oil increased. Compared with the single-slug injection mode, it was easier to recover the clustered remaining oil by B-PPG flooding under the multi-slug alternating injection mode.