The relationship between stent expansion conditions and clinical outcomes is not completely understood. This prospective cohort study included patients who were successfully implanted with second-generation drug-eluting stent in 2012 and received follow-up angiography in 9-12 months. Stent over-expansion was defined as \ge 1.05 of the stented segment over the reference artery diameter. Imaging parameters were measured, and the follow-up period was 7 years. A total of 123 patients with 161 lesions were enrolled, and 75 (46.58%) stents were found to be over-expanded. The baseline clinical and procedural data were comparable. Stent over-expansion showed a markedly increased diameter stenosis percentage (DSP) at 1-year follow-up (24.12 \pm 21.10% vs. 14.65 \pm 16.75%, P = 0.002) and high late lumen loss (LLL) in-segment (0.54 \pm 0.62 mm vs. 0.31 \pm 0.55 mm, P = 0.014). Furthermore, 63 patients with \ge 1 over-expanded stented lesions were classified into the over-expansion group. Cumulative major cardiac adverse event (MACE) was higher in the over-expansion group than the norm-expansion group (17.5% vs. 8.3%, P = 0.133). Target lesion revascularization/target vessel revascularization increased during the 7-year follow-up period in the over-expansion group compared with the norm-expansion group (11.1% vs. 3.3%, P = 0.098). The Kaplan-Meier cumulative MACE-free survival showed a better tendency for statistical differences in the norm-expansion group than in the over-expansion group (log-rank test; P = 0.083). Conclusion: Stent over-expansion is associated with a significant increase in LLL and DSP at 1-year angiographic follow-up and with the increasing trend of cumulative MACE during 7-year clinical follow-up period compared with stent norm-expansion. Stent over-expansion needs to be avoided.