Blast loads acting on structural components under close-in explosions are nonuniform; therefore, the damage to structural members under such blast loads is local and the failure mechanism is more complex as compared with that of distance explosions. Because of the lack of an accurate blast load model, a precise damage analysis method, an efficient damage assessment method, and practical protective measures, the blast-resistant design of structural members under close-in explosions faces great challenges. In this paper, a fluid-solid coupling numerical method is proposed and then used to investigate the failure mechanism of reinforced concrete (RC) columns under close-in explosions. The results show that local failure, such as crushing, cratering, and spall of concrete, is more likely to happen when the RC column is subjected to close-in blast loads. Parametric studies are carried out to investigate the influences of column dimensions and the reinforcement ratio on the damage degree of RC columns. Based on the numerical results, a rapid damage assessment method is also proposed and validated in this paper. The results show that the proposed method can efficiently evaluate the damage degree of RC columns and can be used in the blast-resistant design of RC columns to protect against close-in explosions.