As a sustainable approach to wastewater treatment, microalgae have been extensively used to degrade antibiotics. However, the underlying mechanisms involved in the degradation process remain unclear. Therefore, this study investigated the biotransformation mechanism of sulfathiazole (STZ) by Chlorella sorokiniana (C. sorokiniana) at the molecular level. The results show that C. sorokiniana could efficiently degrade STZ, achieving a maximum degradation rate of 94.74 %, mainly through biodegradation routes. Transcriptome analysis has elucidated the potential biological transformation mechanisms driving the degradation of STZ by microalgae, focusing on the uptake, translocation, and biotransformation as key metabolic processes. In particular, STZ induced the up-regulation of genes associated with cell adhesion, membrane protein, and lipopolysaccharide, suggesting their involvement in the uptake of STZ by microalgae. Furthermore, ABC, MATE, and MFS transporters were identified as crucial for the transmembrane transport of STZ by microalgae. A plausible biotransformation pathway for STZ degradation was proposed, identifying hydroxylation, oxidation, ring cleavage, and formylation as the primary transformation processes. The up-regulation of key enzymes such as monooxygenases, dioxygenases, hydrolases, and transferases suggested their pivotal role in the biodegradation of STZ. This research provides valuable insights into the biotransformation mechanisms of STZ by microalgae, thereby laying a theoretical framework to advance the implementation of microalgae in the treatment of antibiotic-contaminated wastewater.