Spaced repetition has been widely investigated as an effective technique to enhance human learning. However, a limitation of previous research is that most previous studies on spaced repetition in the English as a foreign language (EFL) learning contexts were dominated by in-class implementation and relied on small-scale controlled experiments. To fill this gap in the literature, in the present study, we conducted a large-scale natural experiment on spaced repetition using data from a digital game-based learning (DGBL) application, Dawn of Civilization. Specifically, we examined learners’ natural learning repetition patterns in the game app and their effects on the forgetting rate of the learned words. The data were analyzed based on the identified repetition patterns, and differences in the forgetting rate of these patterns were analyzed. The results revealed that learners who closely followed the recommended spaced repetition schedule of the game had significantly lower forgetting rates, indicating better knowledge retention.