This study investigates the syntactic complexity in English prose between the writings of Chinese scholars and the corresponding translations generated by AI-based machine translation systems. A corpus of 100 English abstracts written by Chinese scholars and 300 English abstracts translated by ChatGPT 4.0, Google Bard and Microsoft Bing was constructed. These texts were analysed using 14 measures of syntactic complexity as defined by the L2 Syntactic Complexity Analyzer (Lu, 2010). The analysis revealed that when comparing the original Chinese-English texts with the outputs of machine translation systems, significant differences were found in 13 of the 14 syntactic measures. Conversely, when comparing the translations from ChatGPT 4.0, Bard and Bing, significant differences were found in 10 of the 14 measures. This research advances the understanding of machine translation systems and has relevant implications for pedagogy and assessment in the field.