The undesirable formation and low-efficient decomposition of Li2CO3 on cathode of Li-air battery, is an insurmountable obstacle for its practical applications. High efficient cathode catalysts toward Li2CO3 decomposition still have not been thoroughly studied. Here, we report a porous MnO prepared by heating precursor using a facile ethanol refluxing method and employed as high efficient cathode catalyst in ambient Li-air battery for Li2CO3 decomposition. Compared with the pure carbon electrode, it demonstrated an improved electrochemical performance of more than 100 cycles in ambient air and low charging overpotential. At the 100th cycle, the discharge/charge terminal voltage of MnO cathode remains 2.65/4.00 V, which is much higher/lower than that of pure carbon electrode (2.36/4.44 V). These results reveal that the superior catalytic performance of MnO towards the Li2CO3 decomposition during charging. Additionally, we carefully examined the time- and depth-dependent discharge products distributions in the carbon-containing cathode, especially for the formation of Li2CO3, by X-ray photoelectron spectroscopy. The Li2CO3 was initially formed on the surface of Li2O2, and then gradually into the inner part of the Li2O2, which reveals that the Li2O2 is inclined to react with CO2 rather than the carbon based electrode materials. These results present essential information for the development of high efficient cathode catalysts toward the high-efficient decomposition of Li2CO3 for practical Li-air batteries in ambient air.