The method for determing methenamine in yuba was set up. The sample was extracted by acetonitrile. Methenamine was hydrolyzed into formaldehyde by hydrochloric acid. The formaldehyde can be reacted with 3-methyl-2-benzothiazolinone ketone hydrazone hydrochloride (phenol reagent) and ferric ammonium sulfate to form a blue-green compound, which can be measured by spectrophotometry at 630 nm after degreasing by neutral alumina. Some effect factors of experimental conditions were investigated like the amount of hydrochloric acid, hydrolysis temperature, adsorbent category and adsorption condition, reaction time and so on. The result showed that the determination was not interfered by fat, sugar, protein and phenylalanine, while slightly interfered by sulfite, which did not affect the determination conclusion. The method showed a good linearity at the range of 0.6～6.0 μg/mL of methenamine. The average recovery was 88.2%. The method is sensitive, and can be used to detect methenamine in yuba.