Given the importance of epistemic modal use in academic writing, this study aims to investigate how well Chinese PhD students use English epistemic modals and whether their achievement in using English epistemic modals is correlated with their degree of Tolerance of Ambiguity (TOA) as a learning style. Fill-in-the-blank exercises were designed to test the participants’ use of epistemic modal verbs in English. Based on theories about matching, embedding and form-function mapping in second language learning, this study analyzed the errors that participants made when they chose modal forms in English to express epistemic functions/meanings conveyed by Chinese modal verb “会” (hui) in different syntactic contexts. A questionnaire survey was conducted to measure the participants’ degree of TOA and IBM SPSS 26.0 was used for statistical calculation of the correlation coefficient between the participants’ scores of TOA and their scores in using epistemic modal verbs. It was found that the degree of TOA is significantly correlated with epistemic modal use, i.e., the less tolerant of ambiguity a participant was, the less successfully he/she did in using epistemic modal verbs. The results of this study carry implications for language teaching and indicate the necessity of enhancing acquisition of epistemic modality and raising students’ tolerance of ambiguity.