The long-term safe operation of a dam can be influenced by the ageing of anti-seepage curtains under its foundation. Based on a synthetic analysis method, basic ageing patterns and some indexes of identification are presented. Three kinds of patterns, including a relatively stable pattern, a slow-weakening pattern and a weakening in stage pattern were classified in terms of the relative anti-seepage efficiency. Each pattern has its unique anti-seepage efficiency during the analysis time. Three types of indexes can be used to identify the ageing. The first type is the micro dynamic elements of seepage, including Ca²⁺, TDS, pH, inorganic and organic matters, which can be used to reveal the ageing mechanisms. The second one is the macro dynamic elements, including the uplift pressure and flow rate, which can be used to identify the ageing degrees and the basic aging patterns. The third type is the numerical analysis indexes, including the saturation index and ratios of the different seepage sources, which can be used to quantify the ageing process. Evaluation systems for the aging behavior of anti-seepage curtains under a concrete-dam foundation can be constructed based on the 3 types of identification indexes.