This study investigates the impact of metacognitive knowledge on vocabulary learning among English as a Foreign Language (EFL) learner, involving 776 university students in China. Its primary goal is to develop and validate a scale for assessing metacognitive knowledge in vocabulary learning. The scale is structured around three sub-dimensions: person, task, and strategies, identified through exploratory and confirmatory factor analyses. These sub-dimensions serve as independent variables in analyzing their influence on incidental vocabulary learning outcomes from reading, which includes knowledge of word form, meaning, and use. The study’s results validate the scale and reveal that the three factors – person, task, and strategies – have varying impacts on learners’ incidental vocabulary learning performance. The findings emphasize the crucial role of metacognitive knowledge in EFL vocabulary acquisition, offering insights for enhancing learning strategies. Additionally, the study provides important implications for educational practices in vocabulary teaching and learning.