The effects of current pulse width on Micro-arc Oxidation (MAO) process for AZ31B magnesium alloys were investigated using pulse power source. The pulse shapes were recorded by an oscillograph. The thickness of MAO coat was measured by an eddy current thickness gauge. The surface and cross section morphologies of MAO coat were observed by scanning electron microscope (SEM). The energy consumption of MAO process was calculated by curves of voltage-time. The results indicate that with current pulse width increasing from 15 μs to 90 μs, the arcing time during MAO process shortens from 358 s to 25 s, the arcing voltage first drops and then increases. When pulse width is 30 μs, the arcing voltage reaches the minimum value of 183 V. The pore diameter of Millipore is enlarged but the quantity is reduced. And the thicker ceramic coat is obtained on Mg samples but the density of MAO coat is decreased. The energy consumption of arcing process is decreased at first and then raised with increasing of pulse width, and the minimum value is 3.9 kJ when pulse width is 30 μs. The energy consumption of MAO coat growing process is nearly of fold increase with increasing of pulse current. The energy consumption per thickness of MAO coat drops firstly and then rises stably, and the minimum value is 10.2 kJ when pulse width is 30 μs.