To deal with structures with uncertain parameter which often lacks sufficient statistical information in damage detection, the uncertain parameters were modeled as interval numbers. The structural strain modal testing method was introduced using finite element method, and the strain modal was further expressed as the function of the interval parameter. Combined with the first order Taylor expansion and interval analysis, the range of each strain modal under parameter uncertainty is estimated, and thus we obtained an approach for structural damage detection considering uncertainty. Comparison between the proposed method and the traditional probabilistic method was conducted. Numerical simulations of a simply supported beam under different damage were performed by investigating the change of strain modes due to different damage levels and uncertainty levels. The influence rules of damage and uncertainty on strain modal were obtained, and a brief discussion of the impact of noise on damage detection was performed. The work of this paper provides a new technique and thought for damage detection of uncertain structures.