Objectives Neutrophil-to-lymphocyte ratio (NLR), one of the composite biomarker of systemic inflammatory status, was proved promising in predicting clinical outcomes of acute coronary syndrome (ACS). However, there were no evidences that NLR was directly relative to the clinical outcomes of unstable angina pectoris (UAP). Therefore, this study was aimed to detect whether NLR could predict the coronary artery lesion severity (indicated as SYNTAX score) and clinical outcomes (especially long-term cardiovascular mortality) in patients with. Methods In the single-centre retrospective study, 4110 patients with UAP were enrolled and divided into two groups according to their primary NLR values and followed up at a median time duration of 36 months. The differences of SYNTAX score and cardiovascular mortality between groups were analysed, and the predictive value of NLR was determined. Results NLR was positively and linearly correlated with SYNTAX score (r = 0.270). Diabetes (p = 0.049), lymphocyte (p = 0.004), NLR (p = 0.002) and SYNTAX score (p < 0.001) were independent predictors of long-term cardiovascular mortality in patients with UAP. Kaplan-Meier analysis revealed higher occurrence of cardiovascular mortality when NLR > 2.38 (p = 0.015). Receiver operating characteristic (ROC) analysis showed that NLR = 2.76 is an effective cut point for predicting cardiovascular mortality (69.2% sensitivity, 64.8% specificity). Conclusions NLR value was positively related to the severity of coronary artery lesion and proved to be an independent predictor of cardiovascular mortality in patients with UAP. This study would contribute to therapy and prognosis optimisation of UAP.