Abstract: Although p-hydroxybiphenolcolorimetry has applied in detecting the content of lactic acid in rumen fluid,the disadvantage of its traditional method need consume for a long time and operate troublesome processes.The experimental method was optimized based on traditional theory,and the reaction process could be completed within one hour.Results showed that the content of lactic acid at 10-100 μg/mg and absorbance had a good linear relationship,and R2 was 0.998 7,and the recoveries were between 97% and 105%.This method is fast and has high accuracy,so it is suitable for the analysis of lactic acid in rumen fluid.