Objective To analyze the expression of complement C3a receptor 1 (C3AR1) in low-grade glioma (LGG) tissues and the association with the prognosis and immune cell infiltration in LGG based on The Cancer Genome Atlas (TCGA) database. Methods The gene expression profile data and clinical information of 514 LGG patients were downloaded from TCGA database. The expression levels of C3AR1 in LGG and normal tissues were compared. The expression levels of C3AR1 in different World Health Organization (WHO) grades of LGG tissues and their relationship with prognosis were analyzed. The influence of clinical features and C3AR1 expression level on prognosis of LGG patients was analyzed. The correlation between C3AR1 expression in LGG tissues and tumor immune cell infiltration, and the influence of immune cell infiltration level on prognosis were analyzed, and gene groups positively correlated with the expression of C3AR1 were further analyzed by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment. Results The expression of C3AR1 in LGG tissues was significantly higher than that in normal tissues (P<0.05). The expression of C3AR1 was higher in WHO gradeⅢ LGG tissues than in WHO gradeⅡ tissues (P<0.05). The prognosis of C3AR1 low-expression group was significantly better than that of the high-expression group (hazard ratio=1.7, 95% confidence interval 1.1-2.4, P=0.003 6). Patient age, LGG grade and expression level of C3AR1 were independent prognostic factors of LGG (all P<0.01). The expression level of C3AR1 was positively correlated with the immune cell infiltration level of LGG (all P<0.01), and the latter was related to the prognosis of LGG patients. Gene groups positively related to C3AR1 regulated several LGG-related key pathways:interferon γ-mediated signaling pathway, conserved immunoglobulin (Ig)/major histocompatibility complex (MHC) site, Ig-like Cl type domains, MHC class Ⅰ/Ⅱ-like antigen recognition protein, specific regions of junctional peptide, and Toll-like receptor signaling pathways. Conclusion C3AR1 is associated with the prognosis and immune cell infiltration of LGG, and it can be a biomarker for grading diagnosis, immunotherapy and prognosis of LGG.