The invention provides a method for preparing a carbon steel chlorohydric acid pickling corrosion inhibitor. According to the method, prepared coarsely-extracted apple pectin is modified through a subcritical water method, and the carbon steel chlorohydric acid pickling corrosion inhibitor is obtained. The molecular weight of the coarsely-extracted apple pectin is modified by adjusting the reaction conditions of a supercritical water method, and the optimal corrosion inhibition efficiency is achieved. The method is simple and easy to conduct, the corrosion inhibition efficiency is high, pollution to the environment is avoided both in the production process and the application process, the practical value is achieved, and the method is suitable for popularization and application.