The invention discloses a water channel circulating system for water sampling and belongs to the technical field of automatic water samplers. The system comprises manual valves, electric ball valves, a two-position two-way solenoid valve, Y-shaped tee joints, a cross joint, peristaltic pumps, even mixing barrels, overflow pipes and a water sample analyzer. The system is characterized in that a water sample is divided into a branch I and a branch II after passing through a pipeline sampling device, and the branch I and the branch II are connected with a residual liquid draining system, the water sample analyzer, a reserved sample output bottle, an air blowback system and a clear water cleaning station respectively through the Y-shaped tee joints. A water channel circulating sampling technology for water sampling is adopted as follows: 1) water sample collection; 2) water sample shunting; 3) water sample analysis; 4) water sample reservation; 5) pipeline cleaning. Water sampling, sample reservation and analysis processes as well as the final water channel cleaning process are set reasonably, so that the automatically collected water sample is reserved correspondingly while analyzed, the circulating water channel can be cleaned timely after analysis and sample reservation, and the detection of the water quality is powerfully guaranteed.