The invention belongs to the technical field of production of modified starch, and particularly relates to a paper strengthening agent, and a preparation method and application thereof. The preparation method of the paper strengthening agent comprises the following steps: mixing waxy rice starch, distilled water and an expansion inhibitor into starch suspension; dropwise adding aqueous alkali into the starch suspension to alkalify after the components of the starch suspension are evenly mixed; slowly dropwise adding a cationic etherifying agent solution to the alkalified solution, and carrying out heating reaction; cooling at room temperature after the reaction; dropwise adding acetic acid to neutralize, carrying out vacuum filtration, washing by an ethanol solution, drying an obtained solid mater, and crushing after drying so as to obtain the paper strengthening agent.