Previous research is seldom addressed to biliteracy relationship for adult learners in the foreign language context. The present study was a repeated-measure design in a 9-month longitudinal study. One hundred and thirty-nine college students (ages 19-21 years) who learned English as a foreign language were measured twice at pre- and post-tests on a series of measures in the first language (L1) Chinese and the foreign language (L2) English literacy and various L2 literacy-related language and cognitive skills. Results showed first, L1 literacy was relatively stable but L2 English literacy-related skills (except working memory) improved over time. Second, phonological and morphological awareness were two key variables consistently predicting L2 word reading at pre- and post-tests, although different language/cognitive constructs played a role in predicting the dynamic progress in L2 literacy. Further, correlation analysis and hierarchical regression analysis revealed that after controlling grade and gender, L1 literacy and L2 literacy-related language/cognitive skills explained more variances in predicting L2 literacy in post-test. The results indicate the language proficiency in L2 has a modulate effect on the biliteracy relationship. The findings shed light on the relationship between predictors and biliteracy skills.