The invention discloses polyvinyl formal applied to a negative-pressure water seepage device. The polyvinyl formal comprises polyvinyl alcohol which is 1700-1800 in average molecular weight, 37-40% formaldehyde and 36-38% hydrochloric acid, wherein the ratio of the polyvinyl alcohol to distilled water to formaldehyde is 1:(6.25-8.75):(0.5-1); and the polyvinyl formal applied to the negative-pressure water seepage device is prepared by the steps of adding distilled water to the polyvinyl alcohol, heating and dissolving in a water bath at more than 95 DEG C, cooling to 55 DEG C after completely dissolving into a transparent paste, adding the hydrochloric acid which serves as a catalyst, stirring for a first time threshold under a preset rotating speed threshold by virtue of a mechanical foaming method, adding formaldehyde, continuing to stir for a second time threshold, pouring into a die, curing the die in an oven for a third time threshold at 55 DEG C, taking out the cured product in the die, soaking and neutralizing in alkaline liquor to obtain the polyvinyl formal applied to the negative pressure water seepage device. The polyvinyl formal disclosed by the invention can be used for reducing the cost of negative pressure irrigation.