Estrogen receptor α emerged as a well validated therapeutic target of breast cancer for decades. However, approximately 50% of patients who initially responding to standard-of-care (SoC), such as undergo therapy of Tamoxifen, generally inevitably progress to an endocrine-resistance ER+ phenotype. Recently, selective estrogen receptor covalent antagonists (SERCAs) targeted to ERα have been demonstrated as a therapeutic alternative. In the present study, series of novel 6-OH-benzothiophene (BT) derivatives targeting ERα and deriving from Raloxifene were designed, synthesized, and biologically evaluated as covalent antagonists. Driven by the antiproliferative efficacy in ER+ breast cancer cells, our chemical optimization finally led to compound 19d that with potent antagonistic activity in ER+ tumor cells while without agonistic activity in endometrial cells. Moreover, the docking simulation was carried out to elucidate the binding mode, revealing 19d as an antagonist and covalently binding to the cysteine residue at the 530 position of ER helix H11.