The invention provides a pretreatment device of sewage pretreatment system, which has a water inlet and a water outlet. The sewage pretreatment system also comprises a water mixer, a clean water valve and a water quality test device. The invention provides a sewage pretreatment method, which comprises: detecting the pollution index value; judging whether or not the pollution index value of the sewage meets the requirement of the first threshold value; mixing clean water and sewage and adjusting the inflow of the clean water to make the pollution index value of the mixed sewage meet the requirement of the first threshold valve if the pollution index value dose not meet the requirement of the first threshold value according the result of the judgment; and pretreating mixed sewage. The sewage pretreatment system and the method ensure the quality and the stabile flow of the effluent of the pretreatment device.