A sludge dehydrating system and a method thereof based on a thermal hydrolysis technology include: a homogeneous slurry unit, a hydrothermal unit, a flash reactor, a waste heat recovery unit, and a dehydrator; wherein a viscosity of sludge is lowered by homogenously slurrying before entering a pump, which is conducive to transportation; a sludge tank firstly crashes and then quantitatively transports for improving a homogenizing efficiency; a diluent is sludge dehydrated filtrate which is mixed with flash steam in an ejector, wherein an mixing efficiency is high; during homogenously slurrying, waste steam generated is added into a sludge diluent pipe for being absorbed; a hydrothermal unit include a variety of forms such as an intermittent form and a continuous form, in such a manner that reaction parameters, especially reaction time are effectively guaranteed; the flash steam enters the homogeneous slurry unit for heating the sludge.