The adverse impact of a low family socioeconomic status (SES) on rural-to-urban migrant children’s academic achievement has been widely demonstrated. However, knowledge regarding the mechanisms underlying this relationship is limited. The current study aimed to examine the potential mediating effects of educational expectations and the moderating effects of subjective SES on the relationship between family SES and academic achievement among Chinese rural-to-urban migrant adolescents. A sample of 321 rural-to-urban migrant adolescents (48.2% girls; mean age = 11.73 years, SD = 1.16 years) was recruited in China. The results indicated that migrant adolescents’ educational expectations mediated the relationship between family SES and academic achievement. In addition, the link between family SES and educational expectations was weak among adolescents with higher levels of subjective SES. These findings suggest that subjective SES serves as a protective factor buffering the negative effects of a low family SES on migrant adolescents’ academic achievement through educational expectations.