Polymer gels can cause damage to lowly to moderately permeable formations during both oilfield applications and laboratory evaluations of conformance treatment. To enhance reservoir sweep efficiency and oil recovery while using conventional bullhead injection, this study proposed a new injection method where a small amount of branched preformed particle gel (BPPG) was injected as a preflush slug before the injection of in situ cross-linked polymer gel (ISCPG). After that, a gel breaker was pumped to degrade particle gel. The breaking behavior of ISCPG and particle gels in gel breaker was first evaluated using static gel-breaking tests. Core plugging experiments combined with nuclear magnetic resonance (NMR) measurements were then employed to assess the sweep efficiency improvement and oil displacement effects of the ISCPG-BPPG composite gel system. Results showed that injecting the ISCPG-BPPG composite gel system into core samples, followed by a gel breaker, could effectively remove damage at the end faces of lowly to moderately permeable formations. Thus, it expanded the subsequent waterflood sweep volume and thus increased oil recovery by 28.43% original oil in place (OOIP). The gel breaker contained a 2:8 ratio of depolymerizing agent to ammonium persulfate at 2 wt% concentration. Moreover, BPPG in the composite gel system could act as a preflush slug to block subsequent ISCPG invasion into lowly to moderately permeable formations, reducing damage to oil-bearing formations. Compared to the single-gel injection method, the ISCPG-BPPG composite gel system demonstrated superior conformance improvement capability and oil recovery after gel-breaking treatment. The NMR technology was employed to analyze the novel conformance improvement method based on composite gel and gel-breaking mechanisms at the microscopic scale.