Radiotherapy is well-recognized as an efficient non-invasive remedy for cancer treatment. Since 10 Gy, a weekly total dose for conventional radiotherapy, was proven to create unreparable and residual DNA double-strand breaks (DSBs), they were found to give rise to mitotic failure, such as mitotic catastrophe, which resulted in multiple micronuclei associated with premature senescence. We demonstrated that pulverization of micronuclear DNA was caspase-dependent and triggered not ATM-dependent but DNA-PK-dependent DNA damage response, including phosphorylation of histone H2AX. Pulverization of micronuclear DNA and senescence-associated secretory phenotype (SASP) worsen tumor microenvironment after radiotherapy, so that senolytic drug was applied to eliminate senescent cancer cells. Prematurely senescent cancer cells with micronuclei caused by 10 Gy of γ-irradiation were subjected to 5 μM of ABT-263, a Bcl-2 family inhibitor, and selective cancer cell death by apoptosis was observed, while ABT-263 had little effect on growing cancer cells. Western blot analysis showed augmented expression of both apoptotic and anti-apoptotic proteins in senescent cells, indicating that increased apoptotic factors are essential for selective apoptotic cell death in combination with ABT-263. Our results suggested that selective elimination of senescent cells alleviates SASP and micronuclei-mediated the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) activation, both of which lead to unfavorable adverse effects caused by radiotherapy.