Online homework has become an important teaching and learning activity due to the impact of COVID-19 pandemic-related restrictions. This study explored the relationship between primary students' online homework completion and learning achievement. It also investigated the moderating effects of key factors including the role of the students and the involvement of their parents on this relationship. Based on a total of 3,210 Chinese online homework assigned in the Spring of 2020 to fourth grade primary school students in Wuhan, China, hierarchical linear modeling was employed to examine the relationship between primary students' online homework completion and their learning achievement. Simultaneously, the effects of potential moderators including students' information literacy, students' prior academic achievement, parental digital self-efficacy, and parent-teacher partnership were investigated. The results showed a significant positive effect of students' online homework completion on learning achievement. Moreover, students' information literacy, students' prior achievement, and parent-teacher partnership positively moderated this relationship. Based on the findings, practical implications for school administrators, teachers, and parents are discussed herein to promote online homework completion and enhance students' learning achievement.