Objective This study aimed to investigate the influence of intracavitary brachytherapy on immune function and nutritional indicators in patients with rectal adenocarcinoma. Method A total of 89 patients with locally advanced rectal adenocarcinoma admitted to the Oncology Department of Zhongnan Hospital of Wuhan University from May 2016 to July 2024 were included. According to the treatment regimens they were divided into the study group neoadjuvant chemoradiotherapy combined intracavitary brachytherapy n = 33 patients and the control group neoadjuvant chemoradiotherapy n = 56 patients . Peripheral blood absolute lymphocyte counts ALC and their subsets albumin and total protein levels 1 week before treatment and 1 month after the end of radiotherapy were obtained. The effects of different treatment groups on immune function lymphocytes and their subsets and nutritional indicators albumin and total protein levels were compared. Multiple linear regression was used to analyze the influencing factors of lymphocytes and their subsets 1 month after radiotherapy. Result There was no significant difference in albumin and total protein levels between the study group and control group 1 week before treatment and 1 month after the end of radiotherapy. At 1 month after the end of radiotherapy the study group had higher levels of ALC and CD8 + T cells than the control group. Similarly patients with pathologic complete response pCR showed higher levels of ALC and CD8 + T cells than patients with non-pCR. Different treatment groups were identified as independent influencing factors for ALC and CD8 + T cells 1 month after the end of radiotherapy. Conclusion Compared to the neoadjuvant chemoradiotherapy group intracavitary brachytherapy combined with neoadjuvant chemoradiotherapy can increase the number of lymphocytes and CD8 + T cells 1 month after radiotherapy improve immune function and do not increase the risk of malnutrition in rectal adenocarcinoma patients.