The invention discloses test paper for measuring moisture of transformer oil, preparation and an application. The test paper is prepared from a common cellulose paper piece with the cellulose content being 99.1%-99.6% as a carrier by soaking in a color developing agent, wherein the color developing agent is prepared from composition as follows: mixed powder of 38%-40% by mass of copper sulfate, 38%-40% by mass of calcium sulfide and 20%-22% by mass of acid blue which are weighed in an anhydrous environment is dissolved in petroleum ether, dispersed and mixed uniformly. The test paper is light blue and can detect trace moisture in the transformer oil sensitively, the color change process of the test paper is light blue, dark blue, aquamarine blue and black blue in sequence, the test paper is dark blue when the moisture content is 20 ppm, the color changes obviously, the period of validity is long, detection is convenient and visual, manufacture is simple, and the cost is low.