To make full use of cornstalks from biomass waste, a green method for producing high value-added chemical products like organic acid with nickel oxide (NiO) as a solid oxidant was studied. The results show that the liquid product contains formic acid, lactic acid, pyruvic acid, oxalic acid and fumaric acid, etc. The yield of lactic acid was studied primarily. It was found that when the dosage of NiO is 3.5 mmol, the concentration of NaOH is 3 M, the reaction temperature is 230ºC and the reaction time is 3 hours, the yield of lactic reached 15.3%. In addition, the principles behind the conversion of cornstalks into organic acid with NiO was preliminary discussed. These results can offer a new method of developing a sustainable way for the efficient utilization of biomass waste.