The diode pumped alkali rubidium laser is investigated by the three level rate equations for improving the laser output power of diode pumped alkali vapor laser (DPAL). The systemic conditions are considered, including pump power, pump bandwidth, gain medium cell temperature and length. The simulation results are essentially coincident with the existing experiment results. Furthermore, we put forward the optimal design parameters of gain cell for high output power and optical-optical conversion efficiency with a definite pump condition.