Researches show thatquare- with- gap tube has advantages of better hydraulicconditions, broad adaptability for treatment of various raw waters and high sedimentation efficien-cy. A contrast experiment is carried out between squarewith- gap tube and hexagonal tube. The for-mula of hydraulic character parameter is deducetl for square-with- gap tube provides to theoreticalbaqe for practical application of square-with-gap tube.