In seismic risk assessment, seismic fragility analysis is considered as a useful method that can quantitatively give the, structural failure probability for arbitrary earthquake intensity level of interest. As we known, the efficiency of seismic, fragility analysis can be significantly improved by appropriate structural demand models. Although a lognormal model is, currently most used, the rationality of it is still in doubt. In this study, a structural demand model using 3P-lognormal, distribution for generating analytical fragility curves is proposed. The performance of the proposed model is confirmed based on a large amount of structural nonlinear dynamic analytical data. As the result, the proposed model shows a better fitting effect than the existing lognormal model on structural demand distribution and is expected to be more applicable to seismic fragility analysis.