The relationship between socioeconomic status (SES) and academic achievement can both reflect the degree to which students’ academic achievement is affected by SES and reveal detailed information about educational equity in a country/economy. Based on PISA 2018 data, this study investigated the effects of SES and its components on student academic achievement at both the student and school levels in a population of 15-year-old students from Beijing-Shanghai-Jiangsu-Zhejiang (China). Results showed the following: (1) B-S-J-Z (China) students’ academic achievement was positively related to their family SES and school SES. (2) The effects of different SES components on academic achievement varied, and the most predictive variables were the father’s occupational status, the father’s level of education, and the mother’s occupational status. (3) School disciplinary climate was shown to moderate the effects of SES on academic achievement. Drawing on the research findings, this study suggested that the government should (1) further promote education equity with a primary focus on disadvantaged student groups; (2) improve the enrolment policy and increase the diversity of student backgrounds; (3) promote teacher professional development and continuously improve school effectiveness.