A disclosed preparation method for cassava starch modified by combining three kinds of modification processes is characterized by comprising: taking starch as a raw material, employing a wet-process crosslinking-oxidation-esterification chemical modification technological route, specifically, performing a crosslinking reaction and an oxidation reaction on an active starch slurry at the same time, then adding a pretreated composite esterification reagent to perform esterification reaction, and adjusting the pH value of the starch slurry to 5.0-6.5, so as to finish combined modification of cassava starch, filtering to obtain wet starch with the water content of 35% or less, then adding polyethylene glycol, mixing uniformly, and performing drying, cooling, crushing and sieving, so as to obtain the cassava starch modified by combining three kinds of modification processes. Only one time of post-processing process comprising washing, centrifugation, drying and the like is performed, the technological process is continuous and automatic, energy and reaction time are saved; and the product is low in gelatinization temperature, good in formability, improved in permeability, caking property, film forming ability and anti-ageing property, and is suitable for being used as a coating adhesive and a surface sizing agent in papermaking industry, a sizing agent in weaving industry, an industrial binder and the like.