In the digital era, traditional communication has undergone a drastic transformation into computer-mediated communication (CMC), which can be classified into synchronous CMC (SCMC) and asynchronous CMC (ASCMC). This study compared the effects of extracurricular CMC among students about schoolwork on students' digital reading achievement between SCMC and ASCMC, with the categorization of students into three levels, i.e., top, medium, and low achievers. Data from 6,109 samples representing 183 schools in Japan were retrieved from the dataset of Programme for International Student Assessment (PISA) 2018. Due to the hierarchical nature of the data, multilevel regression analysis was adopted to identify the variance of studentand school-level. The results revealed a significant positive effect of SCMC on the digital reading performance of students, while ASCMC had a negative impact. Furthermore, among the three categories of achievers, medium achievers were the group most sensitive to both SCMC and ASCMC at any frequency, while top achievers were not significantly influenced by SCMC and ASCMC at any frequency, and low achievers were influenced by SCMC and ASCMC only at several frequencies. These findings indicate that it is more advisable for students to use SCMC, and treatment should be customized to students of different levels.