In order to obtain high slope efficiency of 2 μm laser output, it is essential to design operating parameters of fiber laser prior to the experiment. Based on the rate equations, the pump and signal power equations of thulium doped fiber laser have been built. The effects of the cross relaxation coefficient K3101, the pump power filling factor Γp, the laser reflectivity of input mirror R3, the laser reflectivity of output mirror R4, and the Tm3+ concentration N of kilowatt level thulium doped fiber laser on the slope efficiency have been theoretically analyzed by an improved shooting algorithm. The results indicate that the slope efficiency decreases with the increasing laser reflectivity of output mirror, the pump power filling factor. Meanwhile, the slope efficiency increases with the increasing laser reflectivity of input mirror, the doping concentration and the cross relaxation coefficient, but the doping concentration and the cross relaxation coefficient have more obvious influence on the slope efficiency compared with the laser reflectivity of input mirror.