The Vietnamese government has implemented many changes to improve the English competencies of the Vietnamese people. However, the results seemed unsatisfactory because the English as a foreign language teachers had been affected by many external factors. Thus, this study addressed two main research questions, including (1) What external factors affect EFL teachers' responses to English language teaching pedagogical reforms? and (2) How these factors affect different groups of teachers in light of their backgrounds? This study was designed as a mixed-methods approach using a questionnaire and semi-structured interviews to answer those questions. The current study involved 102 EFL high school teachers working in the Southwest of Vietnam. The results showed that the factor on "students' learning outcomes" was the most influential external factor. Besides, the influence level of external factors was differed by the teachers' educational qualifications in the influence level of some external factors, namely "previous educational policies," "students' learning outcomes," and "school facilities." At the end of this paper, some critical discussions, educational implications, limitations, and recommendations for further research are presented.