The invention relates to a method for preparing beet alkaline aqueous solution with high concentration and high pureness and good fluidity at relatively low temperature (2 DEG C), which is produced according to the steps that: firstly, vegetable oil is amidated with alkyl tertiary amine; then alkyl amido tertiary amine, the amidated product, is quaternized with halogenated carboxylic acid; wherein, during the quaternization period, the pH value of the reaction system is controlled to ensure that the quaternization is more complete and micelle thickener is added during the reaction; a further post-processing reaction is carried out after the quaternization, so that the residue of the halogenated carboxylic acid is less than 20ppm, thus meeting the requirements of high concentration and high pureness.