The radiative transition probabilities A JJ′ , nonradiative transition probabilities ω JJ′ and fluorescence lifetimes τ J of excited levels of Er 3+ ions in YLF crystal are calculated by using the Judd Ofelt theory. On the basis of equations of fluorescent dynamics the populations w J of excited levels, dependented on doped concentrations x, are calculated too. For 4I 11/2 → 4I 13/2 transition (at wavelength of 2.7 μm), only if doped concentration is bigger than 3 at.% the population inversion can be, and the fluorescence strength and doped concentration are increased. But for 4I13/2→4I15/2 transition (at wavelength of 1.73 μm), there is optimal doped concentration for which the fluorescent strength is maximal. When the pump probability is 103～105 s-1, the xop is 1.8～3.0 at.%. The spectral characteristic parameters of Er3+ ions in YAG crystal are calculated. Based on the comparison the advantages of Er3+:YLF as a laser crystal are discussed.