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This study aims to investigate the effects of the dynamic increase factor for beam structures under conventional blast loading. Combined with differential equations of an equivalent single-degree-of-freedom (SDOF) system in elastic and plastic response stages, the dynamic increase factor expression of a beam structure under conventional blast loading was derived. Based on the degree to which the structure reached the plastic stage, the dynamic increase factors under different ratios of equivalent mass-load transformation factors in the elastic stage and elastoplastic stage were analyzed. Using simply supported beams, simply supported and fixed beams, and fixed-end beams as the types of analysis beams, nine calculation conditions of the dynamic increase factor were completed. The calculation results were compared with the formula used by the Chinese blast-resistant design code. The results showed that for a beam structure design with a large ductility ratio ß (ß > 2), a ratio of the equivalent mass-load transformation factors in the elastic and plastic stages larger than 1 would increase engineering costs. Compared with simply supported beams and simply fixed beams, the relative error caused by the different ratios for the equivalent mass-load transformation factors is smaller for fixed-end beams. The existing research ignores the equivalent mass-load transformation factor in the plastic stage and takes the ratio of the equivalent mass-load transformation factors in the elastic and plastic stages as 1. This will have a significant impact on the dynamic increase factor of simply supported beams, and the value of maximum error is approximately 18%. If the equivalent mass-load transformation factor in the plastic stage is ignored, then the error in calculating the dynamic increase factor will be small for beam structures with lower ductility ratios ß (ß < 1.6), and the value of maximum error is approximately 5%. [ABSTRACT FROM AUTHOR]