The invention discloses a device for treating sludge water in a water supply plant. The device comprises a tank as well as a rapid mixing zone, a contact flocculation zone, a slow push flow zone, a concentrated sludge zone, a sloping plate sedimentation zone, a supernatant zone, an ultrafiltration zone and a clear water zone which are arranged in the tank, wherein an electromagnet is arranged in the concentrated sludge zone, and a water collecting tank is arranged in the supernatant zone; a water outlet of the water collecting tank points to the ultrafiltration zone, an immersed hollow fiber membrane is arranged in the ultrafiltration zone, a water outlet of the immersed hollow fiber membrane points to the clear water zone, and the clear water zone is connected with a water outlet pipe. The device is compact in structure, high in integration degree, high in water load, energy-saving and environment-friendly and occupies small area, the effluent quality is good, and the utilization rate of sludge water is high. Besides, the sludge water quality characteristic is fully combined, original alum flocs are used, magnetic sepiolite is added, the contact fluctuation effect is good, and flocs are uniform, compact and can settle easily. Meanwhile, the electromagnetic separation technology is introduced, the sludge and water separation speed is high, and the sludge concentration is high.