We investigated the effect of a plasma charge separation field on radiation reaction (RR) in laser–plasma interactions. By implementing the plasma field equation developed in one-dimension into the classical Landau–Lifshitz formula, our numerical test-particle modeling revealed distinctive plasma electron dynamics in three regimes: the low density underdense regime, the near-critical density (NCD) regime and the highly overdense one. It is discovered from the electron trajectories and gamma-ray emission that the RR effect is mostly significant at near-critical plasma densities. The underlying mechanism is well interpreted by the theoretical analysis from the perspective of the potential contained in the charge separation field and demonstrated in two dimensional particle-in-cell simulations using the quantum electro-dynamic photon emission approach. Our findings indicate that the NCD plasma is the most efficient medium to leverage RR and generate high-energy and low-divergence gamma-photons.