The present paper reports a novel method for the determination of silicon content in straw fibrous material black liquor based on α-Si—Mo heteropoly acid spectrophotometry. The selected conditions were as follows: detection wavelength 360　nm, pH 4.0, and reaction time 10　min. It was found that the acidic soluble lignin in the sample liquor was the major interference species in the silicon content determination. The interference of acidic soluble lignin can be eliminated by hydrogen peroxide-nitric acid digestion method. The present method is not only simple, rapid, stable and less interferential, but also of good measurement precision and accuracy, with the relative standard deviations of 0.9%, and recoveries of 99.0%～102%. It is suitable for use in high silicon content of black liquor routine rapid analyses.