The invention provides a slaked lime feeding system in a water processing system. The slaked lime feeding system comprises a lime powder bin. An outlet of the lower end of the lime powder bin is provided with a feeding machine. The feeding machine is connected with a metering conveying machine. The metering conveying machine is connected with a feeder though a pipeline made of transparent materials. The feeder is connected with a lime milk solution. The lime milk solution tank is provided with an automatic sand draining valve and communicated with a lime milk feeding water tank through a lime milk chemical feeding pipe. According to the slaked lime feeding system, through the pipeline made of the transparent materials, the whole process of material conveying and flowing can be observed, and dredging can be conducted conveniently and timely once blocking is found; due to the fact that the automatic sand draining valve is arranged, automatic sand draining can be conducted regularly; the labor intensity of workers is lowered, deposition and blocking caused by sand grains are reduced, the concentration of a lime agent prepared through a limb chemical feeding device keeps stable, the error does not exceed +/-0.5%, precise control over the PH value of feeding points can be guaranteed, and the error does not exceed +/-0.3.