Objective To explore the absolute counts and relative proportion of total lymphocyte count and lymphocyte subsets of pediatric patients with malignant solid tumors and observe the effects of chemotherapy. Methods Clinical data of 112 pediatric patients with malignant solid tumors were retrospectively analyzed. The peripheral venous blood was collected before first and second course of chemotherapy and blood routine indexes, immunoglobulin, absolute count of lymphocyte, and lymphocyte subtypes were detected. Results Various types of lymphopenia appeared when pediatric patients developed solid tumors for the first time. The major types were CD8+ lymphopenia (36.3%) and CD4+ lymphopenia (31.1%). The differences of pediatric patients with different types of solid tumors were not statistically significant (P=0.08). Compared with pediatric patients with normal lymphocyte counts, pediatric patients with lymphopenia were prone to severe infections after chemotherapy (19.1% vs 5.9%, P<0.01), especially for pediatric patients with neuroblastoma (23.9% vs 6.4%, P<0.01). After chemotherapy, the lymphocyte counts of pediatric patients with solid tumors significantly decreased, especially the counts of CD19+ cells and CD4+ cells (P<0.01). The ratio of CD4/CD8 decreased significantly (P<0.01). Conclusion Compared with health children, the incidence of lymphopenia among pediatric patients with solid tumors is high. The lymphocyte count decreases significantly after one course of chemotherapy, especially B lymphocytes and CD4+ T lymphocytes. The humoral immune is impaired early. More attention should be paid to lymphocyte proliferation and humoral immune supplement, which may be helpful for preventing infections after chemotherapy.