Pancreatic ductal adenocarcinoma (PDAC) is highly aggressive, with KRAS mutations, particularly KRAS G12D, present in the majority of cases. While development of KRAS G12D inhibitors is rapid, acquired resistance remains a major clinical challenge. Therefore, in this study, we aimed to evaluate the efficacy of novel KRAS G12D inhibitor HRS-4642, investigate its potential resistance mechanisms, and overcome resistance through combination therapy. The results demonstrated that although HRS-4642 showed potent and selective anti-tumor activity, acquired resistance has been observed in a clinical trial (NCT06520488). Through integrated analyses in vitro (HRS-4642-resistant cell lines), in vivo (HRS-4642-resistant patient-derived xenograft (PDX)), and in clinical samples (HRS-4642-resistant patients from NCT06520488), we identified reactivation of the unfolded protein response (UPR) via the MEK/ERK pathway as a key resistance mechanism. To overcome this, we performed a comprehensive drug combination screen and found that nimotuzumab synergized strongly with HRS-4642. This combination effectively enhanced efficacy and reversed resistance both in vitro and in vivo (cell-derived xenograft (CDX) and PDX). Mechanistically, while the addition of nimotuzumab was validated both in vitro and in vivo to sustain the suppression of the MEK/ERK/UPR axis, it concurrently exacerbated endoplasmic reticulum stress in vitro, thereby driving profound cellular damage. Our findings not only reveal a clinically relevant resistance mechanism to KRAS G12D inhibition but also provide a rational, effective combined strategy. Ultimately, the combination of HRS-4642 with nimotuzumab offers a promising therapeutic strategy for PDAC patients harboring KRAS G12D mutations, laying a foundation for advancing clinical research in overcoming resistance to KRAS G12D-targeted therapies.