<p>Syntactic complexity constitutes a salient indicator of second language (L2) learners’ developmental progress and writing proficiency; however, empirical findings remain inconclusive, largely due to insufficient consideration of contextual variables such as genre and rating levels. The present study sought to elucidate the moderating effects of genre and rating levels on the predictive relationship between syntactic complexity and human evaluations of students’ English as a foreign language (EFL) writing quality. A convenience sample of 411 Chinese-speaking college English as a foreign language (EFL) learners was recruited from two colleges in Western China. They were asked to write an argumentative and a narrative essay, and syntactic complexity of their written productions were analyzed by the Tool for the Automatic Analysis of Syntactic Sophistication and Complexity. The results of ANOVA and Welch’s Test demonstrated statistically significant differences in syntactic complexity between argumentation and narration and also across three rating levels. Multiple group analysis showed syntactic complexity indices demonstrated different predictive influences on human ratings of EFL students’ argumentative and narrative essays across different rating levels and genres. The results of these analyses also revealed that genre and rating levels could moderate the above-mentioned predictive effects. These findings highlight the need to consider genre and proficiency levels in validating rating scales and rater training, and they underscore the potential role of syntactic complexity as an indicator in L2 writing assessment.</p>