Background: The role of molecular residual disease (MRD) detection in non-small cell lung cancer (NSCLC) patients after definitive resection has been progressively elaborated. However, the dynamic patterns of circulating tumor DNA (ctDNA) during and after radical radiotherapy (RT) remain unclear but are critical for detecting MRD in this context. Methods: We enrolled 139 patients with locally advanced NSCLC treated with radical RT in this prospective study. And 761 blood samples were successfully detected by ctDNA-MRD assay at different preset timepoints before, during and after RT. Results: There was a declining trend in ctDNA concentrations of “during RT” time point (RT reached 40 Gy) relative to those after RT. Patients with negative ctDNA of “during RT” and “after RT” time points may indicate an early response to RT, and had better progression-free survival (PFS) than those with late response to RT(ctDNA clearance only at “after RT” time point, HR = 0.30, 95%CI: 0.12-0.75, p = 0.004). Moreover, patients with both negative ctDNA of “during RT” and “after RT” time points could not benefit from consolidation therapy (HR = 0.72, 95%CI: 0.29-1.78, p = 0.478), while patients with any positive ctDNA of them could (HR = 0.59, 95%CI: 0.38-0.93, p = 0.023).The sensitivity of longitudinal MRD was 97.8% and only two patients with local progression were overlooked. The 2-year cancer-specific PFS of patients with longitudinal undetectable MRD was 88.4%, which corresponds to the potentially cured population. For patients with only blood-informed MRD detection, the sensitivity and median lead time of longitudinal MRD were 97.7% and 3.9 months, respectively, which indicating the feasibility and efficacy of blood-informed MRD detection strategy. Conclusions: These data shed new light on the ctDNA-MRD detection for localized NSCLC patients who received radical RT.