Provided is a use of SERD with a CDK4/6 inhibitor and a PI3K/mTOR pathway inhibitor. In particular, provided is a use of a selective estrogen receptor downregulator (SERD) in combination with one or more selected from a cyclin dependent kinase 4/6 (CDK4/6) inhibitor, phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamyein (mTOR) pathway inhibitor in preparation of a medicament for treating breast tumors.