The energy transfer and cooperation upconversion processes are investigated in Yb³⁺/Er³⁺ codoped phosphate glass. Based on the measured curves of output power versus incident power, the laser and spectroscopic parameters of the glass are fitted and analyzed. We focus on the resonant energy transfer constant k from Yb³⁺ to Er³⁺ as well as the cooperation upconversion coe±cient C_up from ⁴I_13/2 of Er³⁺. The fitted k and C_up can give almost the same results for different thicknesses of glass disk with the same doping concentrations. The determination of these parameters is helpful for the development of Yb³⁺/Er³⁺codoped laser glass.