Abstract: The irreducible tensor methods of ligand field theory has been applied to analyse the fluorescent spectrum of Tb (Ⅲ) in KTbP₄O₁₂The optimized parameters, 5 crystal field parameters and 8 relativistic parameters, were obtained based upon both the approximate C₄v symmetry of the environment of Tb (Ⅲ) and the experiment magnitude of the spectrum. The result of calculation is agreement with experiment very well.