Aiming at the non-probabilistic characteristics of the factors influencing dam service reliability, a non-probabilistic Info-gap model is established to quantifiably describe the non-probabilistic uncertainty of the influence factors. A non-probabilistic reliability analytical method of dam service performance is proposed based on two non-probabilistic reliability measures, Info-gap decision theory and the volume ratio. Non-probabilistic target reliability indexes of dam service performance are then preliminarily determined. The proposed non-probabilistic method is used to analyze the structural service reliability of a dam. The results of the reliability analysis in various function modes demontrste the engineering applicability of the method and the varified rationality of the determined non-probabilistic target reliability as well.