Abstract: Based on the debugging operation of paste backfill system in a copper-zinc mine in Harbin，reasons of clear water polluted by feeding slurry，low concentration underflow，and pressure rake during debugging operation were pointed out.Corresponding solutions including reformation of feedwell，flocculant dosing points and underflow circulating pipes etc.were introduced.It has been proved that：①It is beneficial for creating a suitable condition for stable and sufficient mixing of tailings and flocculant by choosing proper selection of the tailings dilution mode and decreasing the kinetic energy of slurry entering feedwell；②Increasing properly and arranging reasonably the flocculant dosing points can improve flocculation effect and underflow concentration of deep cone thickener；③Reasonable design of the underflow circulating pipes allows for smooth flow of underflow，reduce running resistance of rake and decrease the accident of pressure rake.

Key words: Paste backfill, Deep cone thickener, Pressure rake, Flocculation