Teacher efficacy has been demonstrated to be highly related to teacher retention, which emphasizes the importance of the study of first-year teachers' efficacy. Although teacher efficacy has been studied in regard of pre-service teachers and in-service teachers, research about it concerning first-year teachers, who are in a challenging stage is lack of attention, and teacher efficacy displayed through the first public lesson worth much exploration since teachers' performance in the significant first public lesson has a strong impact on their lasting teaching career. This study is a qualitative study involving two first-year teachers of secondary school through the first public lesson in Southwest China. The study adopts the psychological sources of information postulated by Albert Bandura as analytical framework. Research tools include recorded classroom observation, semi-structured interviews, teacher diaries as well as reflection journals. The results show that the level of first-year teachers' efficacy is changing and is highly related to the academic performance of the students assigned to the teachers. The study also shows that teacher efficacy is positively affected by the first public lesson, which is regarded as an effective professional activity in the study. Besides, teachers’ relationship with students and teachers' previous teaching experiences are emerged as two additional important sources in explaining first-year teachers' efficacy in Chinese context.