The invention discloses a preparation process of 2-hydroxyimino-N-phenyl-acetamide. The process includes the steps that a sodium sulfate aqueous solution with a certain concentration is added into a vessel, then hydroxylamine sulfate is added, the pH value is adjusted with dilute sulfuric acid, then the reactor is placed in an ultrasonic instrument, the ultrasonic device is turned on, a certain temperature is controlled, trichloroacetaldehyde is added, ultrasonic treatment is performed for a certain time, then phenylamine is added for a certain time of ultrasonic treatment, then the reactor isplaced in a heating jacket, heating is performed until boiling, and cooling, filtering and washing are performed to obtain the product. Compared with the prior art, the process has the advantages that the yield of the reaction product can reach 90%-95%, which is increased by 5-10 percentage points; the sodium sulfate concentration can be decreased by 5-10 percentage points; due to the acoustic cavitation effect of ultrasonic waves, the reaction conditions are simplified, the reaction velocity is increased, and side reactions are reduced.