Despite extensive use of online language learning during the pandemic of COVID, there is insufficient research on what factors influence students' foreign language achievements in online learning conditions. This article investigated the roles of language aptitude and online self-regulated learning in foreign language achievement in mainland China. 76 freshmen from two classes at a university in Jiangxi participated in this study. They were required to complete an aptitude test of MLAT and a questionnaire on online self-regulated learning. The results showed that: (1) The students' language aptitude is at a relatively low level, and their online self-regulated learning is at an intermediate level; (2) A positive correlation is detected between language aptitude, online self-regulated learning, and their English achievements (r=0.621 & 0.583 respectively); (3) Language aptitude alone (grammatical sensitivity and associative memory ability) accounts for 38.9% variance in English achievement. Language aptitude and online self-regulated learning contributed 52.4% of the variance to their English achievements. Overall, the findings of the study confirm the high predictive power of the MLAT and predictions of the Linguistic Coding Deficit Hypothesis (LCDH) advocated by Sparks and colleagues. Pedagogical implications are also discussed.