In the water quality pretreatment system, the activated carbon filter can absorb the residual chlorine that cannot be removed in the pre-stage filtration to prevent the reverse osmosis membrane from being oxidatively degraded, and also adsorbs pollutants such as small molecules and organic substances leaking from the previous stage. Water odor, colloid and pigment, heavy metal ions, etc. have obvious adsorption and removal effects, and also have the effect of reducing COD. The SDI value of the RO influent can be further reduced to ensure SDI < 5 and TOC < 2. Oppm.