The nanocrystalline Cu protective coating was obtained on the Nd-Fe-B magnet matrix in the cyanide-free electrolyte by the ultrasonic-assisted electrodeposition under high cathodic current density. The coating morphology, crystallite size, microhardness, and corrosion resistance of the Cu coatings were analyzed under different ultrasonic frequencies. Results show that with increasing the ultrasonic frequency, the effective cathodic current density of Cu electrodeposition in the complex electroplating solution system is increased remarkably, and the corresponding current efficiency is enhanced, thereby achieving the dense nanocrystalline Cu coating. Under the condition of the cathodic current density of 4.0 A·dm^-2 and frequency of 40 kHz, the Cu coating with the crystallite of 18.8 nm in size can be obtained. The bonding strength between the Cu coating and sintered Nd-Fe-B matrix is improved via ultrasonic-assisted electrodeposition which promotes the electrodeposition in the blind holes of Nd-Fe-B matrix. The corrosion resistance of Cu-coated Nd-Fe-B materials with the same coating thickness is improved gradually with increasing the ultrasonic frequency.