In an 80 ℃ water bath, Nd3+ can catalyze decoloration reaction of ponceau S by H2O2 oxidation. Based on this, a fading spectrophotometric new method for the determination of neodymium was put forward. The maximum absorption wavelength of the system is 450 nnm and at this wavelength there is a good linear relationship between a concentration of 0.2 ~ 4 μ g / mL of Nd3+ and the absorbance difference. The method has been applied to the determination of neodymium in molecular sieve.