The invention relates to a preparation method of modified bentonite adsorbent used for cationic dye waste water decolouring, and belongs to the field of printing and dyeing waste water decolouring adsorbent preparation. The preparation method comprises following steps: iminodiacetic acid and 3-glycidyloxypropyltrimethoxysilane are taken as raw materials, and are subjected to ring-opening reaction in an aqueous solution so as to synthesize a bentonite modifier; and the bentonite modifier is used for grafting modification of bentonite so as to obtain the modified bentonite adsorbent. The raw materials are easily available; bentonite is cheap; cost is reduced; the preparation method is simple, and is friendly to the environment; no secondary pollution is caused; modification of bentonite with the bentonite modifier is capable of enlarging the space structure of bentonite, providing the modified bentonite adsorbent with large absorption space, introducing carboxylic group ions into the modified bentonite adsorbent, and increasing the absorption capacity of the modified bentonite adsorbent on cationic dye. Desorption of the cationic dyes absorbed by the modified bentonite adsorbent can be realize, and recycling of the modified bentonite adsorbent is realized.