There is an increasing concern on the vulnerability of concrete gravity dams under blast loads. To a better evaluation of the blast-resistance, the effect of the initial stress field is taken into consideration herein. Firstly, a fluid-solid coupling numerical model is established to investigate the blast-resistance of concrete gravity dams. The numerical results indicate that the initial stress field has non-neglectable effect on the shock wave propagation, blast vibration, and failure mode of concrete gravity dams. The damage of dam base may be overestimated without the consideration of initial stress field. Moreover, a parametric study are carried out for further investigations of the dynamic responses and damage properties under various explosion scenarios (explosive charge, detonation depth, and standoff distance). With the consideration of the initial stress field, it can be concluded that the dam head nearest the explosive source is the weakest part, and the safety of the dam base is another important consideration, which may lead to severe dam-break floods. At last, a new method is proposed and recommended to evaluate the blast resistance of the whole concrete gravity dam, where the blast-induced vibration and penetration depth of cracks are suggested for the dam crest and dam base, respectively.