Knowing the factors affecting students' learning achievement in digital learning is a crucial educational issue nowadays. However, recent research has paid less attention to how an individual's internal factors (prior knowledge) influence their learning achievement through cognitive engagement, and previous studies generally employed students' self-reported data, which are subjective. This study investigated the relationships between students' prior knowledge, cognitive engagement, and learning achievement in digital game-based learning by using eye-tracking technology to analyze their visual behaviors. A total of 55 university students volunteered to use the game to learn about programming, during which their visual behaviors were recorded by an eye tracker to investigate their cognitive engagement and visual transition patterns. Their prior knowledge of programming was assessed one week before the game started, while their learning achievement was tested immediately after the game ended. The results of the study showed that: (1) Students' prior knowledge had a moderately positive predictive effect on their learning achievement; (2) Students' learning concentration played a mediating role in the predictive effect of prior knowledge on learning achievement; (3) Students' cognitive strategies moderated the predictive effect of prior knowledge on learning achievement; and (4) Groups of students with different prior knowledge and cognitive engagement adopted significantly different modes of visual transformation in the game. These findings further revealed the complex relationship between learners' prior knowledge, cognitive engagement and learning achievement in the game environments, which would be a good reference for understanding individual differences in the game environment and for designing game-based adaptive learning systems.