A wave generator comprises a seat; a positioning ring a wave generating unit received in an upper side of the seat; the wave generating unit being formed by a flow-steady base and a flow-collecting mask; the flow-steady base having a central platform; an upper surface of the central platform having a plurality of projecting blocks which are annularly arranged; the spaces between projecting blocks being formed as flow channels; an inner wall of the wave generating unit being formed with a plurality of longitudinal slots for draining water; a positioning unit for positioning the flow-steady base and the flow-collecting mask precisely; a filtering web having a plurality of holes which formed as an umbrella shape; a positioning rod extending from a central lower side of the web; and a front end of the positioning rod being formed an enlarged tapered head.