A water purification system comprising a composite filter assembly and a booster pump, wherein the composite filter assembly comprises a housing, and a composite filter defined in the housing, the housing defines a raw water inlet, an initial filtered water outlet, a pressurized water inlet, a pure water outlet, a waste water outlet, and a booster pump, the composite filter comprises a pre-posed filter, a membrane filter, and a post-posed filter, wherein the pre-posed filter and the membrane filter are coaxial and are defined in a manner from outside to inside along a radial direction, and spaced from each other, and the post-posed filter is connected with the membrane filter and located at a downstream position of the membrane filter, wherein the post-posed filter is located upon the pre-posed filter and the membrane filter; the initial filtered water outlet is communicated with an outlet end of the pre-posed filter, the pressurized water inlet is communicated with an inlet end of the membrane filter, wherein the initial filtered water outlet and the pressurized water inlet are both defined at a lower end surface of the housing and wherein the pressurized water inlet is circular-shaped, and located at a center position of the lower end surface of the housing, the initial filtered water outlet and the pressurized water inlet are concentric; and the outlet end of the pre-posed filter is adjacent to the inlet end of the membrane filter, the initial filtered water outlet is an annular opening which surrounds the pressurized water inlet; and the booster pump is located at a downstream position of the pre-posed filter and connected with the pressurized water inlet, wherein the housing comprises an outer housing, and an inner housing received in the outer housing, the pre-posed filter is defined between the inner surface of the outer housing and the outer surface of the inner housing, the membrane filter is received in the inner housing, the post-posed filter is defined at an up end of the inner housing; a lower end surface of the outer housing defines a first opening, the periphery of the first opening extends downwards to form a first annular rib; a lower end surface of the inner housing defines a second opening, a projection of the second opening along an up-down direction is located in the first opening, the periphery of the second opening extends downwards to form a second annular rib, the second annular rib extends into the first annular rib; and an inner circumference surface of the first annular rib and an outer circumferential surface of the second annular rib cooperatively form the initial filtered water outlet, the inner circumference surface of the second annular rib defines the pressurized water inlet. a housing, and a composite filter defined in the housing, the housing defines a raw water inlet, an initial filtered water outlet, a pressurized water inlet, a pure water outlet, a waste water outlet, and a booster pump, the composite filter comprises a pre-posed filter, a membrane filter, and a post-posed filter, wherein the pre-posed filter and the membrane filter are coaxial and are defined in a manner from outside to inside along a radial direction, and spaced from each other, and the post-posed filter is connected with the membrane filter and located at a downstream position of the membrane filter, wherein the post-posed filter is located upon the pre-posed filter and the membrane filter; the pre-posed filter and the membrane filter are coaxial and are defined in a manner from outside to inside along a radial direction, and spaced from each other, and the post-posed filter is connected with the membrane filter and located at a downstream position of the membrane filter, wherein the post-posed filter is located upon the pre-posed filter and the membrane filter; the initial filtered water outlet is communicated with an outlet end of the pre-posed filter, the pressurized water inlet is communicated with an inlet end of the membrane filter, wherein the initial filtered water outlet and the pressurized water inlet are both defined at a lower end surface of the housing and wherein the pressurized water inlet is circular-shaped, and located at a center position of the lower end surface of the housing, the initial filtered water outlet and the pressurized water inlet are concentric; and the outlet end of the pre-posed filter is adjacent to the inlet end of the membrane filter, the initial filtered water outlet is an annular opening which surrounds the pressurized water inlet; and