Aim To quantitatively evaluate the residual lesions of patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI) using the residual SYNTAX score, and to investigate the prognostic impact of this score to predict long-term servival rate after PCI. Methods 782 patients with ACS undergoing PCI between January 2014 to May 2015 from the Heart Center of the First Hospital of Lanzhou University were included. The baseline SYNTAX score(bSS) and residual SYNTAX score (rSS) were calculated before and after PCI, patients were divided into tow groups based upon rSS:rSS=0 complete reascularization group(CR), rSS＞0 incomplete reascularization group (IR). IR group was divided into the following three groups:0＜rSS≤4 low-risk group, 4＜rSS≤8 moderare-risk group,rSS＞8 high-risk group. Major adverse cardiac cerebrovascular events(MACCE) rates (all-cause mortality, myocardial infarction,repeat revascularization and stroke) and cardiac death were followed-up in 14 months. Results 676 cases were followed-up, 106 cases were lost to follow-up. (1) Compared with CR group, each group with IR had higher incidence of 3-vessel coronary artery disease(CAD) and higher bSS, especially the patients of the high-risk group with rSS＞8,they had higher incidence of 3-vessel CAD, hypertension and higher bSS than other three groups(P＜0.05). (2) Compared with CR group and low-risk group, the moderare-risk group and high-risk group had a higher MACCE rate, all-cause mortality, cardiac death rate and repeat reascularization rate(P＜0.05), and there was no statistical difference between CR group and low-risk group(P＞0.05). (3)No event Kaplan Meier survival curves showed that the three groups with IR had lower survival curves than CR group, and the no event survival curve of the high-risk group with rSS＞8 was the lowest(P＜0.001). (4)Multivariable logistic regression analysis showed rSS was a strong independent predictor of varieties of adverse cardiovascular events at 14 months of patients with ACS undergoing PCI, including MACCE rate, all-cause mortality, cardiac death rate and repeat reascularization rate, the ROC curve of rSS and the end point events showed rSS had a good prediction ability to the events described above, and there was no obvious correlation between rSS and postoperative myocardial infarction. Conclusion The rSS quantitatively evaluate residual lesions of the patients with ACS undergoing PCI, it is an independent predictor of varieties of cardiovascular adverse events after PCI for 14 months, and the patients with rSS＜4 have a better prognosis than the patients with rSS＞4, and the patients with rSS＞8 have a poor prognosis.