Background: Socioeconomic status (SES) is important in students' reading development. The mechanism of how they are linked, however, is underexplored. Aims: This research aimed to explore whether reading self-concept mediates the link between SES and reading achievement and whether teacher support moderates the direct and indirect link between SES and reading achievement. Samples: Using PISA 2018 database, we selected 17,346 15-year-olds from 537 schools in B-S-J-Z (China) and the United States (U.S.). Methods: Multilevel linear models were calculated to explore the potential mediation of self-concept and the potential moderation of teacher support between SES and reading achievement. Results: Results reveal that reading self-concept mediated the link between SES and reading achievement at student level both in China and the United States, and teacher support moderated the link between SES and reading self-concept both at student level and school level only in China, but not in the United States. Conclusion: Our research highlights the importance of self-concept and teacher support in improving students' reading achievement, especially for students of low SES, which has important theoretical significance and practical enlightenment for reading instruction.