A water-pollutant separation device, comprising a tank (1a), a water inlet pipe (2a) and a water outlet pipe (3a) respectively communicated with the top of the tank (1a), and a pollutant discharge pipe (4a) communicated with the bottom of the tank (1a), is characterized in that the inner cavity of the tank (1a) is provided with a separation structure (5a) being in such an inverted-barrel shape that has an open bottom, an upper closing cover, a smaller external dimension than that of the tank (1a), a gap from the bottom of the tank (1a), and a spoiler (51a), which is installed onto the water inlet pipe (2a) after which goes through the upper closing cover of the separation structure (5a) in the tank (1a), having a sealed lower-end and a plurality of openings on the side-wall to generate subcritical flow to spoil the water flow.