A cyclic flow device is developed to study the effect mechanism of sine flow with different frequency on filtration system. A differential equation of pore water pressure accordant with the test model is built; and its analytic solution is achieved. Theoretical analyses show that the smaller ratio between the consolidation coefficient of soil and the frequency of cyclic flow leads to the higher hydraulic gradient on the border of soil and the lower hydraulic gradient inside the soil. Test results show that when the period of the cyclic flow is 0.5min

the amplitude of hydraulic gradient near to the border of the soil is nearly double of the average amplitude of the filtration system. When the period of the cyclic flow is 62.5 min

the phenomenon of hydraulic gradient focusing is neglectable. Test results show good accordance with the analytical results. The phenomenon of hydraulic gradient focusing will lead to more filtration border erosion and it will diminish the stability of the filtration system. Proper attention shall be paid to this phenomenon in the filtration engineering.