Factors including the upstream dam breaking time T₁, water depths in front of the upstream dam H₁ and downstream dam H₂, upstream reservoir area S₁ and downstream reservoir area S₂ that can influence the dam-break flood evolution in the downstream reservoir, the initial flood wave height and the initial runup height in front of the downstream dam, are discussed based on physical model tests. The results show that, the evolution process of a dam break flood in the downstream reservoir can be divided into four stages: flood wave forming, flood surging, reflection superposition and oscillation attenuation. The initial runup height of the flood wave in front of the downstream dam decreases with the increase of T₁, H₂, and S₂, while increases with H₁, but it has no obvious correlation with the upstream reservoir area S₁. In addition, the empirical formula of the initial wave height in front of the downstream dam is proposed, and the influence degree of each factor on the initial wave runup height from high to low is obtained in the following order: T₁, S₂, H₁, and H₂.