The fluorescence charecteristic parameters of Tm3+ ions in Tm: YAG crystals for radiative transition probabilities Aij, non-radiative probabilities Wij, cross relaxation probability P, fluorescence lifetimes τ and fluorescence branching rations βij, et al. are caculated. The dependence of the population of 3F4 level on dopant concentration x is studied. The optimum relative concentration xop of Tm3+ ions of 4. 9～ 6. 4 % is obtained,when the range of probablility of excited level by pump light are 102～ 106 sec-1.