Background The present study aims to investigate the prognostic role of systematic inflammatory and nutritional indexes in extensive-stage small-cell lung cancer (ES-SCLC) treated with first-line chemotherapy and atezolizumab. Materials and methods Prospective cohort population involving 53 patients were identified from NCT03041311 trial. The following peripheral blood-derived inflammatory and nutritional indexes, including neutrophil–lymphocyte ratio (NLR), platelet lymphocyte ratio (PLR), lymphocyte–monocyte ratio (LMR), systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), prognostic nutrition index (PNI), advanced lung cancer inflammation index (ALI), and lung immune prognostic index (LIPI) were evaluated. Results The optimal cut-off values of the ALI, LMR, NLR, PLR, PNI, SII and SIRI were 323.23, 2.73, 2.57, 119.23, 48, 533.28 and 2.32, respectively. With a median follow-up of 17.1 months, the 1-year OS and PFS were 56% and 8%, respectively. Multivariate analysis showed that PLR was the only independent prognostic factors for OS among ES-SCLC patients treated with chemotherapy and atezolizumab (HR 4.63, 95%CI: 1.00–21.46, p  = 0.05). K-M analysis showed that the OS and PFS for patients with high PLR (> 119.23) were significantly poorer than these with low PLR (≤ 119.23) ( p  = 0.0004 for OS and p  = 0.014 for PFS). In external validation set, prognosis of patients with high PLR was also significantly poorer than these with low PLR in terms of OS ( p  = 0.038) and PFS ( p  = 0.028). Conclusion Pre-treatment PLR could serve as a valuable independent prognostic factor for ES-SCLC who receive chemotherapy and immune checkpoint inhibitors. Further, prospective studies are still needed to confirm our findings.