The invention discloses a preparation method of cross-linked hydroxypropyl composite modified starch by a one-step process. The method, considering that both cross-linking and etherification reactions is performed under high temperature and alkaline conditions, synchronously implements the cross-linking and etherification reactions, and comprises the following steps of: adding a swelling inhibitor, a cross-linker and sodium hydroxide into de-ionized water by certain mass percent, stirring and dissolving, adding starch to obtain a starch emulsion, and alkalizing for a certain duration; and increasing temperature, adding a starch etherifying agent of a certain mass percent, neutralizing, washing, centrifuging, hydrating, drying and sieving after the etherification reaction to obtain the cross-linked hydroxypropyl composite modified starch. The preparation method, by synchronously implementing the cross-linking and etherification reactions, shortens independent cross-linking reaction time, prolongs cross-linking reaction time, reduces dosage of a cross-linker, and lowers production cost and energy consumption of products.