Reactive bright blue praseodymium dyestuff was prepared by using reactive bright blue and praseodymium oxide. The spectra of reactive bright blue praseodymium and dyed silk cloth by reactive bright blue praseodymium dyestuff were studied by UV-Vis and IR spectra respectively. In the range of 200-800 nm, reactive bright blue has four absorption peaks, and lambda(max) is 259 nm; reactive bright blue praseodymium has three absorption peaks, while lambda(max), is 264.00 nm. In the range of 420-760 nm, reactive bright blue has two absorption peaks at 661.50 and 625.50 nm, respectively, and lambda(max) is 661.50 nm; reactive bright blue praseodymium has only one absorption peak at 618.00 nm. Coordinate bond links reactive bright blue to praseodymium ion. Reactive bright blue praseodymium increases linking radicals as compared with reactive bright blue.