Objective To investigate the inhibitory effect of Lycium barbarum polysaccharide (LBP) on the tumor growth and metastasis in MMTV-PyMT mouse model of breast cancer. Methods The population of MMTV-PyMT transgenic mice was expanded and identified. 8-week old MMTV-PyMT-positive female mice were randomly divided into LBP group and control group, 8 mice in each group. The mice of LBP group were given LBP treatment (50 mg/kg, i.p.), and the control group was given normal saline in the same volume, once every 2 days for 4 weeks. The tumor size was measured every two days. The mice were killed at 4 weeks after treatment, the lungs were removed and fixed in Bouin's solution to observe the number of metastatic nodules, and tumor tissues were used for immunohistochemical examination of tumor cell proliferation and vascular density. Results The tumor formation rate was 100% in the MMTV-PyMT-positive mice. The tumor weight of LBP group was 4.208±0.4463 g, significantly lower than the 6.477g±0.3724 g in the control group (P<0.005). The number of pulmonary nodules of the LBP group was 12±1.155, significantly less than that of the control group (20±2.745) (P<0.05). The immunohistochemical examination using Ki67 and CD31 staining showed that tumor cell proliferation and microvessel density of the LBP group were significantly less than the NS group. Conclusions LBP inhibits breast cancer growth and metastasis through the inhibitory effect on tumor growth and metastasis, inhibition of tumor cell proliferation and angiogenesis in MMTV-PyMT mice. These mice can be used as an ideal model for studies on antitumor drug development for the treatment of breast cancer lung metastasis.