In respect to descent trajectory optimization of stratospheric airships,thermal effects were considered.Basic thermal characteristics of stratospheric airships and energy equations about the film and the internal gas were introduced.Besides,the equations of motion for the airship were described in details.To minimum the flight time,a direct collocation method was adopted to convert the trajectory optimization problem into a parameter optimization problem.In different scenarios,the optimal trajectories were developed by a numerical nonlinear solver,sparse nonlinear optimizer (SNOPT).It is shown that the solutions are greatly affected by thermal effects and solar irradiation plays an important role during descent.