This study attempts to investigate whether there is any significant difference between the anxiety level of mastery avoidance-oriented English for Academic purpose (EAP) students and their performance avoidance-oriented counterparts based on their teacher’s supportive motivational discourse during instruction. It also explores the perception of EAP students of the influence of this intervention. This study employs a mix-method design. In the first phase, EAP students were recruited through convenience sampling and then placed into one control (n = 31) and two experimental groups (n = 34 for mastery-avoidance and n = 34 for performance-avoidance). The anxiety levels of both groups were measured using a questionnaire before and after a ten-week treatment. In the second phase, to understand the experimental group’s perceptions of the intervention, 20 participants were interviewed, and 10 were selected for stimulated recall interviews. Non-parametric ANCOVA and posthoc comparison tests showed that the mean levels of anxiety in experimental groups were significantly lower compared to that of the comparison group. However, the teacher’s supportive motivational discourse had a stronger influence on the reduction of the mastery-avoidance group’s anxiety. Content analysis of qualitative data also showed that this intervention increased learners’ emotional engagement and self-actualization. This is the first causal study investigating the effect of the teacher’s supportive motivational discourse on the anxiety of EAP students with mastery and performance-avoidance goal orientation. The implications of teachers’ supportive motivational discourse for improving the delivery of lessons in language education are discussed.