The flipped classroom approach has been extensively adopted in STEM disciplines, while little empirical evidence has been found in language teaching and learning, especially in the English for Specific Purposes (ESP) context. This quasi-experimental study aimed to explore how the flipped classroom approach affected non-English majors' self-efficacy, study process, and learning performance in English learning. An 18-week semester-long flipped approach with pre-class materials and in-class interactive activities was implemented in a university ESP class in Taiwan. The results showed that the experimental group attained significantly higher self-efficacy, and outperformed the control group on the examinations. Moreover, the flipped approach promoted learners' higher-level skills and enabled them to scrutinize new ideas critically and connect them to their existing cognitive structures, and make numerous associations between ideas.