Objective To study the tumorigenic properties of MMTV-PyMT breast cancer transgenic mice at different weeks of age and the changes in the composition of immune cells in the tumor microenvironment. Methods Eight groups of 4, 6, 8, 10, 12, 14, 16 and 18 weeks old MMTV-PyMT female mice (FVB mice as the background) and one group of 8-week-old FVB female mice were prepared for routine blood testing, the pathological changes of the mammary gland and lung metastasis were observed by histopathological sections, and the immune cells in blood, spleen and tumor were analyzed by flow cytometry. Results MMTV-PyMT mice showed adenular ductal lesions at 4-6 weeks of age, and the ductal portion grew to the growth boundary at 8-9 weeks of age, and then gradually broke through the glandular boundary to form early breast cancer at 8-12 weeks of age, and advanced breast cancer at 10-14 weeks of age. At 12 weeks of age, metastases were visible in the lungs of some mice, and at 14 weeks of age, the number of metastases in the lungs increased significantly. With the increase of age of mice, the number of white blood cells, neutrophils and platelets in the blood of mice increased gradually, while the lymphocytes and erythrocytes showed a gradual downward trend. Flow cytometry showed that with the increase of age, the proportion of T cells in the spleen and tumor of mice gradually decreased, the MDSCs in the blood, spleen and tumor gradually increased, and the NK cells in the tumor also gradually increased. Conclusions This study conducted an analysis of the blood routine, pathology, and immune cells in the tissues of MMTV-PyMT mouse models of different weeks of age, providing a novel perspective on the dynamic alterations of the tumor immune microenvironment during the malignant progression of breast cancer.