Task design features have different effects on second language (L2) production and can be adopted for different pedagogical purposes. However, the synergistic effects of task features were left unexplored in the extant task-based literature. The present study investigated the synergistic effects of two task design features, namely, prior knowledge and reasoning demands, on the writing performance of Chinese learners of English as a foreign language (EFL). Fifty EFL learners were invited to complete two writing tasks, with varying reasoning demands, under one of two conditions, namely, with prior knowledge available or without prior knowledge available. Their written texts were analysed in terms of complexity, accuracy, fluency, and communicative adequacy, to reflect the multi-componential nature of task performance. The results revealed that increasing reasoning demands reduced syntactic complexity. Furthermore, the availability of prior knowledge resulted in greater lexical sophistication. The findings also showed that the interaction of prior knowledge and reasoning demands led to substantial effects on lexical diversity, lexical sophistication, and communicative adequacy. These findings are interpreted in light of Skehan's Limited Capacity Model and Robinson's Cognition Hypothesis. Suggestions are provided for the direction of further research on the influence of task complexity on EFL writing performance.