Radiotherapy is under investigation for its ability to enhance responses to immunotherapy. However， the mechanisms by which radiation induces anti-tumour T cells remain unclear. We show that the DNA exonuclease Trex1 is induced by radiation doses above 12-18 Gy in different cancer cells， and attenuates their immunogenicity by degrading DNA that accumulates in the cytosol upon radiation. Cytosolic DNA stimulates secretion of interferon-β by cancer cells following activation of the DNA sensor cGAS and its downstream effector STING. Repeated irradiation at doses that do not induce Trex1 amplifies interferon-β production， resulting in recruitment and activation of Batf3-dependent dendritic cells. This effect is essential for priming of CD8 T cells that mediate systemic tumour rejection (abscopal effect) in the context of immune checkpoint blockade. Thus， Trex1 is an upstream regulator of radiation-driven anti-tumour immunity. Trex1 induction may guide the selection of radiation dose and fractionation in patients treated with immunotherapy.