Abstract: Fish soup, using large yellow croaker and pickled potherb mustard as raw materials, was prepared by traditional boiling method and ultrasonic-assisted boiling method, respectively. The changes in flavor characteristics of fish soup during the boiling process were analyzed and compared by sensory evaluation, high performance liquid chromatography, electronic tongue and electronic nose techniques. The results showed that ultrasonic-assisted boiling significantly increased the contents of soluble protein, TCA-soluble peptides and amino nitrogen in the pickled potherb mustard-large yellow croaker soup, and the sensory scores of the soup in terms of umami, aroma and taste were all better than those of the traditionally boiled samples. Ultrasonic-assisted boiling obviously increased the total contents of flavor nucleotides, organic acids and free amino acids in the fish soup, reaching the maximum values of 57.99 mg/100 mL, 95.22 mg/mL and 121.02 mg/100 mL, respectively, after 90 min of boiling. The contents of sweet amino acids such as proline and serine in the ultrasonic group samples increased, while the contents of some bitter amino acids such as histidine, isoleucine and valine reduced. Electronic tongue analysis indicated that the bitterness of the ultrasonic group samples decreased, while the umami characteristics were significantly enhanced. Electronic nose analysis could effectively distinguish the odor differences between the ultrasonic and control group samples. In summary, ultrasonic-assisted boiling can effectively promote the release and degradation of flavor precursors in fish meat and potherb mustard, and reduce the formation of undesirable flavor components, thereby improving the overall flavor quality of the fish soup.