This article presents the results of the development of technology for obtaining a thick extract of milk thistle fruits using ultrasonic treatment of plant material and extractant in the soaking stage. Materials and Methods: For the research, crushed fruits of milk thistle from “Biokor” Ltd, Penza, Russia, series 011216 were used, and the shelf life is 2 years. The particle size of the plant raw material is 0.5–1.0 mm. An aqueous solution of ethanol of 70% by volume was used as an extractant. Ultrasonic influence on the plant material and extractant were carried out under the following conditions: “Bandelin SONOPULS HD 3200” installation, ultrasonic frequency of 20 kHz, sound system of extracting system 5, 10, and 15 min, emitter power 280 W, and temperature 25°С. Results and its Discussion: It was found that with the ultrasonic treatment of raw materials and extractant in the soaking stage, the output of the phalavolignan complex almost doubles. To achieve the maximum concentration of flavonolignans in the extract, 5 min of ultrasound exposure to a mixture of raw material and extractant. Conclusions: The application of ultrasound at the stage of soaking plant material in the technology of obtaining a thick extract from the fruits of milk thistle is justified. It was shown that under the proposed conditions for extraction of flavonolignans from milk thistle, ultrasound promoted an increase in the yield of flavonolignans from plant raw materials and did not adversely affect these biologically active substances in the extract.