The invention relates to a water purifier unit capable of being combined and expanded freely, which comprises filter cores, filter caps, brackets and tubular connection parts. The bracket is provided with two side plates which are respectively arranged at the outer sides of the water inlet and the water outlet of the filter cap. The side plate is provided with a positioning hole through which the tubular connection part passes exactly. The two ends of the tubular connection part are respectively inserted into the water inlet and the outer outlet of the adjacent filter cap and are fixed. The pipe wall of the tubular connection part is provided with a pair of limit grooves along the circumferential direction. The distance between the two limit grooves is approximately twice as much as the thickness of the bracket side plate. The adjacent two side plates of the adjacent water purifier unit bracket are located and blocked between the two limit grooves through a blocking ring, which realizes the fixing of the bracket and the tubular connection part. The invention fixes the adjacent side plates of the adjacent brackets together through the blocking ring limit groove structure and strings the adjacent filter caps into one body through the tubular connection parts, consequently the connection and fixing of the water purifier units is realized. The fixing manner has smart design. The grade of the water purifier can be expanded and reduced and the operation is simple for the user.