A prognostic nutritional index (PNI) developed by nutritional status and inflammation are closely associated with poor prognosis in malignant tumors. However, the predictive impact of PNI in patients with malignant pleural effusion (MPE) remains inconclusive. The study aimed to determine the predictive value of PNI in prognosis and spontaneous pleurodesis among patients with MPE.The patients diagnosed with advanced non-small cell lung cancer (NSCLC) with MPE in West China Hospital between January 2015 and December 2022 were reviewed and allocated randomly to development set(60%) and validation set(40%). After collecting clinical data, peripheral blood inflammation indices and calculating systemic inflammation indices, the effects of PNI on prognosis and spontaneous pleurodesis have been evaluated by Cox proportional hazards models, Kaplan-Meier method and Nelson-Aalen cumulative risk curve.In total, 261 patients diagnosed NSCLC with MPE were selected (development set: n=157; validation set: n=104), of whom 58.2% were aged <65 years, 53.6% were male and 95.8% were diagnosed with adenocarcinoma. The dichotomous cut-off value for PNI was 44.1, respectively. Compared with lower PNI (PNI<44.1) cases, patients with higher PNI (PNI≥44.1) showed significantly longer overall survival (36.5 vs 24.3 mon, P=0.02) and higher incidence of spontaneous pleurodesis (P=0.009). According to the multivariate Cox analysis, higher PNI was associated with good prognosis and successful spontaneous pleurodesis (P<0.05). According to the results of Cox regression analysis, the PNI-prognosis and PNI-spontaneous pleurodesis models are determined, the receiver operating characteristic (ROC) curves are drawn, and the area under the curves (AUC) value of development set are 0.694 (95%CI: 0.620-0.776) and 0.673 (95%CI: 0.590-0.737).PNI is a reliable biomarker of prognosis and spontaneous pleurodesis in patients with MPE. Attention to the patient's nutritional status and inflammation may improve the prognosis and efficacy of pleural effusion.