The invention discloses a two-component gradient structure non-woven fabric for water filtering and a preparation method thereof. The non-woven fabric is composed of a first-layer fiber web and a second-layer fiber web which are bonded together, wherein the first-layer fiber web and the second-layer fiber web are each composed of trefoil PET filament fibers and circular COPET filament fibers which have different finenesses. The preparation method comprises the steps that PET slices and COPET slices which are subjected to pre-crystallization are fully dried; fusion overstocking, spinning, cooling, drafting and separation lapping are conducted through two groups of different spinning assemblies, then the obtained two layers of fiber webs are overlapped to form a composite fiber web, and finally the composite fiber web is subjected to hot air bonding and melting to be reinforced into a whole. According to the two-component gradient structure non-woven fabric for water filtering, the advantages of different fineness filtering in a filter material can be fully exerted, the filtering efficiency is guaranteed, the service life is effectively prolonged, and pressure drop is controlled within a reasonable range; accordingly, the non-woven fabric has the advantages of being free of rolling points, high in ventilation property and filtering precision, low in pressure drop and long in service life.