Dissolved organic matter (DOM) exists widely in the natural water. It is one of the largest active pools of organic carbon on earth and hence plays an important role in the global carbon cycling. As it contains a large number of chromophores, such as benzene ring, carboxyl, hydroxy, carbonyl, etc., it can adsorb sunlight at certain wavelength regions of the spectrum. A variety of reactive free radicals, such as hydroxyl radicals, singlet oxygen, and hydrated electron, could be generated during this process. These reactive free radicals could play an important role in the photo-transformation and photo-degradation of contaminants in the natural water. In different natural water, the generation of reactive free radicals is very different, resulting the mechanism and pathway of photo-transformation of various contaminats in natural water. This paper reviews the DOM-mediated generating pathways of major reactive radicals, including hydroxyl radicals, singlet oxygen, hydrated electron and excited triplet states of the CDOM. Then influences of them on the photo-transformation and photo-degradation of various contaminants, inculding inorganic ion, methylmercury, polyaromatic hydrocarbon, phenols, and pesticides, etc., in the natural water are discussed. The necessary works in the future research are also briefly outlined.