There was an outbreak of Bacterial Ulceration Syndrome (BUS) of Apostichopus japonicus in two culture ponds located in Dalian, Liaoning Province and Dongying, Shandong Province, in October 2019. Epidemiological investigation detected a kind of free-living turbellarians in the culture system including the body surface of A. japonicus, the water and the sediment. Morphological observation of this species revealed that the body length ranged from 0.96 to 3.26 mm and the body width range from 0.49 to 1.93 mm. The body color was yellow or yellowish-brown. Its head was blunt round shape, with a pair of dark red rodlike ocellus. Two caudal lappets lay coordinately on both sides of its tail.Microscopic examination revealed that symbiotic Zooxanthella within the epidermis and its body was covered with cilia. It was hermaphrodite, with two genital openings behind the mouth. The 18S rDNA gene sequence showed 99.64% identity with Heterochaerus australis. Based on the morphological and genetic analysis, it was identified as H. australis. The ecological study showed that this species was photophobic, its suitable temperature was 18-24 °C, its suitable pH was 5.5-8.0 and its suitable salinity was 20-40. This species also showed strong ability of polar regeneration. The co-cultivation experiment of this species and A. japonicus showed that H. australis can adsorb on the surface of sea cucumber, causing ulceration on the body wall and eventually the death of sea cucumber. However, there was no parasitic phenomenon in the body cavity, intestine and respiratory tree of the sea cucumber. All the results indicate that H. australis is a new enemy species for A. japonicus in pond culture.