Research on self-efficacy has been a productive field while limited studies have explored language learners' formation of academic self-efficacy. This quantitative study developed an instrument with two questionnaires for assessing English as a foreign language (EFL) learners' formation of self-efficacy and academic self-efficacy. It further investigated the complex relationship among learners' sources of self-efficacy, academic self-efficacy and their English proficiency. The participants were 700 EFL learners at a comprehensive university in China, who took a compulsory language course for improving their overall English proficiency. The results validated the two questionnaires and further confirmed the correlations among the three constructs. It is very striking to see that the 'social persuasion' played the most significant and positive role for predicting learners' academic self-efficacy and all aspects of English proficiency. Moreover, 'physiological states' were found as a negative predictor for explaining learners' achievements in English listening and reading. The research highlights the importance of socio-cultural factors during learners' formation of academic self-efficacy in China, particularly the significant roles played by the influential social agents. Implications are provided for reducing EFL learners' physiological stress and sustaining their academic self-efficacy for improving their language proficiency.