According to the model, which is based on the three-level rate equations, the threshold characteristics of side-pumped Rb vapor laser was fistly analyzed. The nonlinear absorption of a Bar laser side-pumped Rb vapor laser and the pumped laser propagation in the cell were simuiated in this model. In addition, the distribution of small signal gain under the condition of pumped threshold was well described by this model. In an example, the influence of cell′s length, temperature, gas pressure, gas composition, focus length of focus lens and the parameters of resonant cavity on side-pumped Rb vapor laser was analyzed. The result shows that optimizing proportion of helium and methane can decrease threshold power effectively. Threshold power has combined impact of temperature and cell′s length. High window transmittance and no more than 70% output coupling can make threshold keep in a lower level. It will be helpful to design key components and to optimize systems.