The L2 motivation self system (L2MSS) has been extensively researched in a variety of contexts, but few studies have delved into its relationship with international posture and the sustainable development of L2 proficiency in the COVID-19 era. To address this issue, we surveyed 156 English majors in China and analyzed their response data with structural equation modelling. The results show that the ideal L2 self positively predicts the L2 learning experience, while the ought-to L2 self has a negative predictive power. The international posture exerts a positive influence on the ideal L2 self and L2 experience, and a negative insignificant influence on the ought-to L2 self. The L2 experience and ideal L2 self have a positive influence on L2 proficiency, while the ought-to L2 self has a negative influence. Our results not only contribute to the generalizability of L2MSS but also deepen the understanding of possible contextual variations of L2 motivation, as well as the uniqueness in the sustainable development of English majors in China during the COVID-19 pandemic.