A high-strength and highly wet-heat-resistant polyvinyl-alcohol-based fiber--in which the crosslinking agent has hardly been oxidized by the heat at the drawing time upon preparation of the fiber, the crosslinking agent has not exhaled so much at the time of dry heat drawing, and the crosslinking agent has penetrated even inside of the fiber so that not only the surface but also the inside of the fiber has sufficiently been crosslinked--can be obtained by the steps of: preparing a polyvinyl-alcohl-based fiber by spinning the polyvinyl-alcohol-based solution, wet drawing the fiber, applying an acetalization compound of an aliphatic dialdehyde having at least 6 carbon atoms to the fiber, subjecting the fiber which contains above compound to dry heat drawing to a total draw ratio of at least 15, and then crosslinking the drawn filament with an acid under mild crosslinking treatment conditions.