The effects of some sodium phosphates as auxiliary agents were studied on the blockade of hardness for silk degumming. In this work, four kinds of sodium phosphates were tested and the results were obtained through masking effects of metallic ions, difference of pH value and boil-off ratio. The degumming of calcium ingredient was analyzed by means of atomic absorption spectrophotometer and degumming test of cocoon shell was performed in the presence of calcium ingredient and sodium phosphates added to soap solution. In the view of the effects of sodium phosphates on calcium hardness, tetrasodium pyrophosphate(TSPP) and sodium phosphate dibasic(SPD) masked calcium ions more than sodium phosphate monobasic(SPM) and sodium hexametaphosphate(SHP). SHP and TSPP have excellent abilities of masking ferrous ions. The pH values of TSPP solution is higher than others, but lower than soap solution. The pH values were differently measured one another among the sodium phosphates but the boil-off ratio was increased in case of sodium phosphate with high pH value.