During the COVID-19 pandemic period, a growing number of learning activities are taking place in online contexts. Along with the adversity in the online course of target language learning, student engagement has been considered important to improve learners' academic achievements of the target language. Although there has been a growing interest in the relationship between students' perceived social support and their online learning engagement, the literature lacks an in-depth investigation of the intricate relations between these two constructs in an online English as a foreign language (EFL) learning setting during the pandemic. To address this gap, this study attempts to develop a model which depicts the relationships between students' perceived social support and their online English learning engagement. A total of 615 university students in China were invited to take part in the study. By conducting structural equation modeling, the results confirmed the mediational model in which behavioral engagement completely mediated the relationships between social support (teacher support and peer support) and three other types of student engagement (cognitive, emotional and social engagement). These findings suggest designing effective instruction and developing support strategies in online teaching to enhance EFL learners' engagement during the COVID-19 lockdown.