While academic emotions have long been a critical topic of research in education, there has been relatively little focus on L2 writing and even less on master's students' L2 writing in in the context where English is the medium of instruction. The current study aims to examine the academic emotional experiences of six participants during thesis writing and the impact of academic emotions on their writing process from the perspective of positive psychology. Drawing from multiple sources of data, including writing materials and semi-structured interviews, this study revealed that thesis writing was a process that was heavily influenced by academic emotions, and the participants reported experiencing four different types of emotions: positive activating, positive deactivating, negative activating, and negative deactivating. The study also found that throughout their learning experiences, their emotions underwent complex and dynamic changes, and the academic emotions experienced by participants posed various effects on their writing process. This study provides pedagogical insights regarding how to help students develop emotional resilience to successfully complete their theses writing.