Abstract: In order to obtain a green and efficient preparation process for carboxymethyl porous starch, carboxymethyl porous starch was prepared by mechanical activation solid phase etherification method, where enzymatically hydrolyzed porous cassava starch was used as raw material, sodium chloroacetate as etherifying agent, and sodium hydroxide as catalyst. Single-factor experiments were conducted to explore the effects of various factors on the degree of substitution (DS) of carboxymethyl porous starch, and the application of carboxymethyl porous starch in soy sauce was discussed. The results showed that the optimal process conditions for the preparation of carboxymethyl porous starch by mechanical activation and solid phase synthesis method were as follows: The molar ratio of porous starch to sodium chloroacetate was 1:1, the mass fraction of sodium hydroxide was 18.8%, the ball milling time was 1.5 h, and the reaction temperature was 50℃. Under these conditions, the highest DS of carboxymethyl porous starch obtained was up to 0.2532. Furthermore, the carboxymethylation reaction of porous starch was confirmed by infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). With the increase of DS of carboxymethyl porous starch, its cold water solubility, water absorption, and adsorption of lemon yellow increased. When DS was 0.2532, the cold water solubility of carboxymethyl porous starch reached 64.94%, the water absorption rate reached 180.73%, and the adsorption amount of lemon yellow reached 2.5086 mg·g⁻¹. The powder soy sauce prepared by carboxymethyl porous starch has better solubility, lower moisture absorption, and higher content of amino acid nitrogen, and is closer to the original soy sauce compared to those prepared by cassava starch and porous starch. Therefore, the mechanical activation and solid phase etherification method can effectively prepare carboxymethyl porous starch, which is simple to operate, environmentally friendly, and highly substitutive, providing a scientific basis for the development and utilization of porous starch.