This paper describes the research on the chemical cleaning process route for the No. 2 once-through boiler of the ultra-supercritical unit in Shanghai Shangdian Caojing Power Generation Co., Ltd. The cleaning process uses 3%‒6% organic composite acid (2%‒4% hydroxyacetic acid +1%‒2% formic acid) for chemical cleaning of the No. 2 boiler. The scale removal rate, average corrosion rate, and total average corrosion amount all comply with the standards of DL/T 794-2024 Guidelines for Chemical Cleaning of Boilers in Thermal Power Plants.