The invention relates to a shaft seal of a centrifugal 
coal slurry pump of a 
coal water 
slurry gasification device. For overcoming the defect that a mechanical seal of a centrifugal low-pressure 
coal slurry pump in the prior art is not long in service life and cannot completely prevent external flushing water from flowing into the pump, three to five spacing rings are added on a back blade and blades of an auxiliary 
impeller of the shaft seal to weaken harmful radial flows in a flowing channel and increase a blocking lift of high-
viscosity coal slurry. An external area of an auxiliary 
impeller cavity is a large-specific-gravity 
coal slurry ring, an internal area of the auxiliary 
impeller cavity is a small-specific-gravity oil ring, and the interface of the two areas is obvious and stable. PLAN23 single-end-face mechanical seal and an improve liquid seal tank are adopted to enable the auxiliary impeller cavity, a mechanical seal cavity and the liquid seal tank to be communicated, it can be monitored that the 
coal slurry is away from the mechanical seal in a central 
control room and a field, long service life is ensured, and external flushing water is not needed completely during normal operation. The shaft seal is suitable for the centrifugal coal 
slurry pump of which the conveying 
solid content is 58-61 wt% and the 
viscosity is 700-3000 mPa.s and is also suitable for other high-
viscosity centrifugal pumps with high conveying 
solid content.