A filter assembly for a fluid filter includes a plurality of filter units assembled in series, and each filter unit includes: a connecting member, an input member and an output member. The input member of each of the filter units has an extending pipe connected to a connecting pipe of another filter unit, so that the filter units can be easily assembled by directly screwing the connecting pipe of the connecting member of one filter unit into the extending pipe of the input member of another filter unit, without using tools or fasteners. Hence, the assembling cost is relatively reduced.