background: Poria cocos Wolf has been used in traditional East-Asian medicine for centuries to effectively treat various gastrointestinal disorders such as diarrhea for its tonic, anti-fungal and anti-bacterial activities. Previous studies have revealed that the tumor development would induce intestinal microbiota dysbiosis and intestinal barrier dysfunction to the patients with breast cancer. objective: To investigate the effect and the mechanism of ethanol extract of Poria cocos (PC) on intestinal barrier function and intestinal microbiota in the mice with breast cancer. methods: Thirty-six female BALB/c mice were randomly divided into four groups (the normal control, model, PC and positive control group). Intestinal histopathological was evaluated by H&E staining. The difference of the intestinal microbiota in each group was studied by 16S rDNA high-throughput sequencing. The level of plasma endotoxin, D -lactic acid (D-LA) and diamine oxidase (DAO) were measured by ELISA. The putrescine content in serum and urine were detected by HPLC. Expression of the tight junction (TJ) proteins, phosphorylated p38 MAPK and ERK1/2 were determined by western blotting. results: Our results showed that tumor development prominently induced the intestinal damage and microbiome dysbiosis in mice. PC prominently remit such histologic damage through enhancing the expression of TJ proteins and decreasing the levels of DAO, D-LA and endotoxin via upregulating the expression of phosphorylated ERK1/2 and p38 MAPK. Furthermore, PC increased the diversity of the intestinal microbiota and strikingly changed the structure and composition of the gut microbiota in the mice by increasing the beneficial bacteria Lactobacillus, Bifidobacterium, and decreasing the sulfate-reducing bacteria Desulfovibrio and inflammatory associated bacteria Mucispirillum, S24-7 and Staphylococcus. Moreover, PICRUSt analysis and the putrescine detection might indicate that PC might be involved in the putrescine metabolism in the mice. Correlation analysis indicated that Prevotella, Rikenellaceae and Bacteroidetes were significantly correlated with Claudin-8 and p38-MAPK expression (p < 0.05). conclusions: PC could improve the dysbacteriosis and repair the intestinal barrier function in the mice with breast cancer. This study provide more data to support the application of PC in breast cancer treatment.