Based on the project of upgrading reconstruction on waterworks in Lai Wu, the hybrid tank of floatation and tube settling was applied to reconstruct the horizontal-flow sedimentation tank for the problem of algae concentration in source water exceeding standard, overloading running, shortened backwashing interval of filter and increased bad odor of outflow. Some main points related to the design of floatation and tube settler were stated. The operation results after reconstruction show the problem of algae-laden water and over-load running can be solved by the hybrid tank in some degree, which provides technology information for real engineering.