This research delves into the examination of the correlation between service learning and critical digital literacy among preservice preschool teachers in Taiwan, with a particular emphasis on the role of online social capital. The survey took place in Taiwan with preservice preschool teachers who willingly completed a questionnaire. Initially, there were 416 data samples for further statistical analysis. This study utilizes literature reviews and theoretical reflections to examine the observed variables and factors in the “Service Learning and Critical Digital Literacy Scales” (SLCDL) questionnaire. To assess these aspects, a Chinese questionnaire is used, covering “service learning”, “critical digital literacy”, and “online social capital”. Using partial least squares analysis, researchers investigate the intricate interplay of online social capital in shaping the nexus between service learning experiences and the advance of critical digital literacy competencies. These results showed that service learning increased critical digital literacy among preservice preschool teachers, and online social capital played a mediating role in this relationship between service learning and critical digital literacy. We should actively encourage preservice preschool teachers to increase their critical awareness by understanding vulnerability and experiencing reality from the perspective of community partners. In addition, we need to provide more service learning opportunities that incorporate online social capital to enhance their critical digital literacy.