The invention discloses a nanometer spherical porous weak light photocatalyst for degrading nitrogen oxide, and a preparation method and an application thereof. The photocatalyst comprises 0.2-35wt% of a titanium source, 64-99wt% of an alcohol solvent, 0.005-0.01wt% of a hydrolysis inhibitor, 0.005-0.01wt% of metal ion-containing deionized water, 5-50wt% of hydroxy group-containing mediate strong acid, and 1-5wt% of inorganic dispersant-containing deionized water, and the hydroxy group-containing mediate strong acid contains SiO2 crystal seeds; and the photocatalyst is a transparent sol liquid of nanoparticles, the particle size of the transparent sol liquid is 10-30nm, the surface of the photocatalyst is spherical porous distribution, and the specific surface area of the photocatalyst is large, and is above 300m<2> / g averagely. The photocatalyst has the advantages of mild production preparation conditions, small production safety risk, easy realization of batch production, weak light catalytic purification and high stability, and solves the problems of strict preparation conditions, low sunshine utilization rate, low air purifying and degrading efficiency, large base material apparent influence, construction inconvenience and the like of present mature photocatalyst products.