Endometriosis is one of the most common diseases that happen in reproductive women. The main symptoms include ovarian endometrial cyst, pelvic pain, and so on. We report a case of a 23-year-old woman with a refractory long-course ovarian endometrial cyst (OEC). The patient was previously identified to have a hypoechoic mass sized 9.7 cm in diameter on ultrasound (US) in the right ovary and was tentatively diagnosed as OEC in another tertiary hospital, who was then subjected to US-guided cyst sclerotherapy while the procedure was failed since only a very small amount of viscous and sticky fluid can be aspirated. The patient was then referred to our hospital for further treatment. Pretreat contrast-enhanced ultrasound (CEUS) showed non-enhancement of the mass with a thin cyst wall and a cyst-in-cyst pattern was observed. The possibility of ovarian malignancy was ruled out and the initial diagnosis of OEC was confirmed. The patient was then subjected to US-guided cyst sclerotherapy with lauromacrogol. The interventional procedure was eventful that no fluid was aspirated as what happened in the previous hospital. Thus urokinase was used to dissolve the old, viscious and sticky blood and finally, all the fluid was aspirated. The total consumption of urokinase was 60,000 U. Then lauromacrogol as a sclerosant was injected into the cyst cavity and the cyst wall was flushed repeatedly with lauromacrogol until the aspirated fluid became light red. Finally, 20 mL lauromacrogol was reserved in the cyst and the interventional procedure cost 2 hours The post-procedure course was uneventful without any discomfort, and the volume reduction rate of the cyst was 54%at 3-month follow-up. The visual analogue scale for the pain decreased from 4 before treatment to 1 after treatment, indicating a successful and effective outcome for the refractory long-course OEC.