We investigated the prognostic value of peripheral serum biomarkers, including albumin-globulin ratio (AGR) and eosinophil-neutrophil ratio (ENR), in patients with advanced tumors treated with PD-1/PD-L1 inhibitors. We also retrospectively analyzed the clinical efficacy of PD-1/PD-L1 inhibition in 95 patients with advanced tumors treated at our center. The prognostic value of baseline AGR, baseline ENR, and baseline neutrophil-lymphocyte ratio (NLR) in the serum were evaluated. We also developed a risk scoring tool to stratify patients based on their prognosis. Univariate Cox regression analysis revealed that age, NLR, Eastern Cooperative Oncology Group (ECOG) performance status (PS), platelet-neutrophil ratio (PLR), ENR, AGR, lactate dehydrogenase levels, treatment line, and treatment type were correlated with progression-free survival (PFS). Multivariate Cox regression analysis showed that age, AGR, ENR, and treatment type were independent prognostic factors for PFS. Patients in the low-risk group had significantly longer PFS than those in the high-risk group. The nomogram concordance index (C-index) was 0.716. Patients with a decrease in AGR of over 20% after the first and second treatment cycles had significantly worse PFS than those without decreased AGR. These findings suggest that baseline AGR and ENR may be useful prognostic biomarkers for patients with advanced tumors treated with PD-1/PD-L1 inhibitors.