As a promising supercapacitor electrode material, NiMn-LDH has attracted great attention due to its high theoretical capacity and easy preparation. However, the development and application of NiMn-LDH in supercapacitors are limited because of its poor cycling stability and low electrical conductivity. To solve these problems, a NiMnMg-LDH with a three-dimensional porous morphology has been successfully fabricated by doping with Mg to improve its electrochemical properties. Experimental results indicate that NiMnMg-LDH-7 delivers a specific capacitance of 1772 F g-1 at a current density of 1 A g-1. Moreover, it can still reach 1080 F g-1 when the current density is increased 10 times, suggesting excellent rate capability. The asymmetric supercapacitor (ASC) NiMnMg-LDH-7//AC can provide a high energy density of 28 W h kg-1 at a power density of 700 W kg-1. Furthermore, the energy density can still reach 16 W h kg-1 even if the power density is increased to close to 3500 W kg-1. The capacity retention of this ASC device can reach 74% after 3000 cycles at a current density of 3 A g-1. These excellent properties of NiMnMg-LDH can be attributed to the obvious improvement of its specific surface area and electrical conductivity owing to doping with the element magnesium. We believe that this work could provide a new idea for the preparation of high-performance electrode materials for supercapacitors.