This study investigates the relationship between socioeconomic status (SES), self-regulated learning strategies (SRLSs), and writing proficiency of complexity, accuracy, and fluency (CAF) in English as a second language writing learning context, using Bourdieu's theory of social practice as a framework. This study's conceptual model specifies that both SES and SRLS positively predict CAF, and SES moderates the relationship between SRLS and CAF. This study collected questionnaire data and argumentative writing samples from 381 Chinese university students and analyzed the data using partial correlation and structural equation modeling. The results showed that SES and SRLS were significantly related to complexity and accuracy but not to fluency. SES significantly moderated the relationship between SRLS and writing complexity and accuracy but not fluency. SES's moderating effects on the relationship between SRLS and complexity and accuracy increased from low SES to middle SES to high SES. SES's moderating effects on both SRLS's and SRLS facets' relation to fluency were complex and mostly nonsignificant. The implications of these findings are discussed, and limitations are noted.