Inadequate research attention has been paid to the learning of a third language. For this reason, this study explores senior English major students' learning of additional foreign languages in seven universities in Shaanxi Province, China. The study examines the relationship between the participants' motivation and language proficiency through a questionnaire, and the collected data are analyzed using hierarchical linear regression analysis. The results identify that the participants' instrumental and integrative motivations positively influence their second foreign language proficiency. Further analysis reveals that the connection between the participants' motivation and language proficiency is mediated by foreign language enjoyment. These findings form the basis of our suggestions for the sustainable learning and teaching of foreign languages in universities.Sustainability 2020, 12, 1302 2 of 13 the field of positive psychology have pointed out the importance of foreign language enjoyment (FLE) in language acquisition [22][23][24], more work is needed to elaborate the role of FLE in foreign language learning. Considering FLE's connection with mental activity, we assume in this paper that it mediates the relationship between motivation and foreign language proficiency, and we try to examine this mediating effect in the TLA context. Therefore, the present study uses survey data from seven universities in Shaanxi Province, China, to explore the process of second foreign language acquisition, examining and explicating the relationship between motivation and language proficiency as well as the mediating role of FLE on the focal relationship. Theoretically, this paper enriches our understanding of how motivation influences second foreign language proficiency, facilitates further interest in TLA in terms of individual differences, and highlights the importance of positive psychology in learning a second foreign language. Practically, it suggests important implications for the sustainability of second foreign language teaching and learning.