In order to improve the electrochemical performance, the phosphate ion doped MnFe₂O₄ (PMFO) was synthesized by a hydrothermal method combined with a subsequent phosphatization treatment. Both the specific surface area and electrical conductivity of electrode material are improved by the phosphate ion functionalization. Furthermore, the specific capacitance is 750 F/g at 1 A/g for PMFO, which is almost 1.7% higher than that of MFO. Besides, the cyclic stability of PMFO electrode is also improved. The asymmetric supercapacitors (ASCs) assembled by positive electrode PMFO and actived negative electrode carbon (AC) display an ultrahigh energy density of 168.8 Wh/kg at a power density of 2.7 kW/kg. The PMFO is demonstrated as a promising electrode material for supercapacitor applications.