Abstract: Leech bites usually lead to more bleeding, and dermal tissue is damaged to form lifelong scars. If leeches enter the body cavity, it will be more dangerous. Therefore, there is an urgent need to develop effective repellents. In this study, oil in water (O/W) emulsion of tea tree oil was prepared with tea tree oil as the main ingredient, and konjac glucomannan (KGM), ethyl cellulose (EC) as the main excipients. The filter-paper ring method and repellent test in water were used to evaluate the repellent effects on leeches. The animal experiments were approved by the Ethics Committee of Academy of Military Medical Sciences and were conducted in accordance with relevant guidelines and regulations. The activities of acetylcholinesterase (AchE), glutathione S-transferase (GST) and carboxylesterase (CarE) in vivo were measured to clarify the repellent mechanisms. The results showed that a uniform and stable tea tree oil emulsion was successfully prepared, which has excellent hydrophilicity and can effectively repel leeches. The tea tree oil emulsion has a good repellent effect on leeches, which can avoid the volatilization of tea tree oil and prolong the effective repelling time. The novel formulation of tea tree oil provides a new idea for repelling effects with long time and high efficiency based on similar essential oil.