The trajectory planning model for stratosphere airship ascent was completed by introducing the net buoyancy calculate method. The objective function and constraints was deduced from the flight characteristic analysis of stratospheric airship ascent process. In addition, the minimum ascent time and minimum energy use were simulated based on the Gauss pseudospectral method. The results show that the minimum ascent time for airship is 2 913.6 s; and if the ascent time is abundant, stratospheric airship can fly to 20 km without exhausting any energy. The simulation results can provide useful reference for stratosphere airship integrated design and argument.