In this study, we designed a flexible online teaching environment (On-Flex Teaching) that allows military adult learners, who have greater difficulties in securing study time regularly than ordinary office workers, to participate in their studies on their own initiative. Their academic participation and academic performance were analyzed through analysis of variance and mediation model analysis. As a result of the analysis, in a flexible online learning environment, the group that implemented the pre-study plan performed more learning activities such as downloading lesson plans, answering class questions, and self-evaluation and had higher academic achievement scores than the group that did not follow the plan. Students who performed flexible, self-directed learning activities by flexibly changing the exam time to suit their own circumstances showed higher academic achievement scores through downloading lesson plans and self-evaluation than the group that followed a prior evaluation plan. In addition, as a result of analyzing the mediation model to determine whether the level of self-directed student participation influences the relationship between the degree of implementation of prior study plans and academic achievement, the results showed that establishing study plans increased self-evaluation scores (p<.001) and downloading lesson plans by increasing the number (p<.05), it was found to have a statistically significant positive effect on the total score. Through these results, this study shows that a self-directed, flexible online teaching environment that enables self-directed learning for military adult learners increases students' class participation behavior, As a result, the significance can be found in the fact that major factors that have a positive impact on academic achievement were identified.