Based on the fact that both the maximum wavelength of the absorption spectrum of May Grunwald′s stain(MGS) and the corresponding absorbance varied when MGS interacted with the large biological molecules of RNA under weak acidic medium,a novel spectrophotometric method for the determination of RNA was proposed. The effects of the acidity of B-R buffer solution,the amount of MGS,the amount of B-R buffer and the time on the interaction of the system were studied in detail and optimized. The decreased absorbance of MGS has a linear relationship with the content of RNA in the range of 0-08 mg/L and the correlation coefficient was 0999 6. The method is simple，rapid,highly sensitive and selective. It was applied for the determination of RNA in synthetic samples with satisfactory result. The reaction mechanism was preliminary studied.