This study applied a quasi-experimental design to investigate the influence and predictive power of learner motivation for achievement, employing a mobile game-based English learning approach. A system called the Happy English Learning System, integrating learning material into a game-based context, was constructed and installed on mobile devices to conduct the experiment. The sample comprised 38 Taiwanese vocational high school students. The experimental period was 8 weeks. Through statistical methods, the results verified the positive effectiveness of the approach for promoting student English learning motivation and achievement, indicating the partially predictive power of student learning motivation on English achievement. Notably, the results evidenced the necessity of a long experimental period and sufficient external stimulation to enhance student learning effectiveness. Finally, this study proposed several positive suggestions for English learning and teaching in Taiwan vocational education.