ObjectiveTo systematically evaluate the composition spectrum of oral microbiota diversity in patients with head and neck tumors undergoing radiotherapy. MethodsComputer searches were conducted on CNKI, CBM, WanFang Data, PubMed, Embase, and Web of Science databases to collect cohort and control studies on changes in oral microbiota in patients with head and neck tumors after radiotherapy. The search period was from January 1, 2015 to June 1, 2025. Two researchers screened and evaluated literature, extracted data, and conducted meta-analysis using RevMan 5.4 software. ResultsA total of 25 studies were included, including 731 patients with head and neck tumors and 275 healthy controls. The meta-analysis results indicated that compared with the control group not exposed to radiotherapy, the detection rates of major oral opportunistic pathogens such as Candida albicans and Streptococcus mutans had significantly increased (OR=3.62, 95%CI 2.23 to 5.89, P<0.01). The Shannon index and Chao1 index for oral alpha diversity had decreased after radiotherapy (SMD=?0.78, 95%CI ?1.23 to 0.34, P<0.01; SMD=?0.64, 95%CI ?1.14 to ?0.15, P<0.001), and the differences were statistically significant. Qualitative analysis showed that after radiotherapy, the relative abundance of Firmicutes, Streptococcus, Lactobacillus, etc. in patients increased, while the relative abundance of Clostridium, Neisseria, etc. decreased. ConclusionThere are changes in the oral microbiota of patients with head and neck tumors after radiotherapy, mainly manifested as an increase in the detection rate of opportunistic pathogens, a decrease in alpha diversity, and an imbalance in the relative abundance of some bacterial phyla. This suggests that oral care should be taken seriously for radiotherapy patients.