In the emission cross section (σemi) calculation of rare-earth ions (RE3+), two parameters, zero-line energy (Ezl) and partition function of lower and upper states (Zl/Zu), are very difficult to be determined. This induces large discrepancy of σemi value, especially for RE3+ with large Stokes shift such as Tm3+. Based on McCumber and Reciprocity theories, we present a transition model and propose a method for Ezl and Zl/Zu determinations of Tm3+ at 2.0 μm emission. Several Tm3+ doped glasses are investigated by this method. Influence of Tm3+ concentration and sample’s thickness to Ezl and Zl/Zu is also discussed. Results indicate that samples with thin thickness are preferred in Ezl determination as well as σemi calculation. Comparison of σemi values derived from F-L equation and our method demonstrates that accurate σemi value of Tm3+ can be obtained by this method. Since the established transition model is a universal one, then this method is suited to all RE3+ singly doped materials theoretically.