It tends to produce high-turbidity raw water during the initial period of typhoons or storms and affect the operation of the purifying system. In this research, the particles size (D98) in raw water was 100-125 μm; that in effluent water was <25 μm. When was under 8 ml/s, effluent water D98 was <10 μm. Flow ratio mitigation and the increase of overflow level can improve the particle removing ratio In aspect of particle size analysis, optimal operating conditions can be achieved setting an 8 ml/sec and 40cm overflow level and D98 grain size can be reduced to 6.25μm as well, particle size removal ratio will gradually climb with the increase of accumulated percentage so that it can reach 70 % for the respective effluent water under D50 and even becomes over 80 % under D98. When under D90, the removing ratio can even go beyond 90 % at an 8 ml/s of flow ratio. Therefore, the proposed pre-treatment reduces high turbidity due to the sudden increase of raw water to the barrier in front of the board.