To comprehensively and systematically study effect of the underflow pipe diameter on various performance parameters of the De-Foulant Hydrocyclone with Reflux Device (DFHRD) and understand its mechanism, we proposed a novel DFHRD with underflow pipe diameter of from 0 to twice apex diameter. We investigated the DFHRD with FLUENT software. Specifically, we used the Reynolds Stress Model and the Discrete Phase Model to capture the 3-D strong swirling turbulent flow and predict the particle motion, respectively. Results indicated that: (i) When the underflow pipe diameter was smaller than the apex diameter (e.g., 0-25 mm in this study), both the separation efficiency and split ratio were proportional to the underflow pipe diameter. This was consistent with the acknowledged effect of the underflow pipe diameter. (ii) On the contrary, when the underflow pipe diameter was larger than the apex diameter (e.g., 25-50 mm in this study), both the separation efficiency and split ratio were inversely proportional to the underflow pipe diameter. Reasons were as follows: at this time, increase of the underflow pipe diameter increased the back pressure and axial velocity in the underflow pipe. As a result, circulation eddies were generated and then increased in the underflow pipe. Besides, range of the optimum underflow pipe diameter does exist (e.g., 5-15 nun in this study). Because, only at this time, we could simultaneously achieve the high separation efficiency (69.14%-78.43%) and acceptable split ratio (0-10%).