The phenoxycyclophosphazene (HPCTP), as a coupling reagent, was introduced into ultra-high-molecular-weight polyethylene (UHMWPE)-magnesium hydroxide (MH) composites to overcome the incompatibility problem. The Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) results confirmed that HPCTP had successfully been introduced into the UHMWPE-MH composites and performed as a coupling reagent. The prepared UHMWPE-MH-HPCTP composites performed promotion of processability, which is indicated by the over 100% increasing of melt flow rate (MFR). The elongation of composites was promoted nearly 600% under the optimized composition of UHMWPE-MH-HPCTP. Although the thermal property was slightly affected, the UHMWPE-MH-HPCTP composites presented significantly enhanced flame retarding ability, which was sufficiently confirmed by 100% promotion of limiting-oxygen index (LOI), 75 s delaying of ignition time and advantageous ash results. By addition of HPCTP, the prepared UHMWPE-MH-HPCTP composites expressed priorities in processability, mechanical property, especially flame retarding ability, and supplied a practical candidate material for the industrial applications.