Aiming at the near space airship, the natural convection characteristics of the helium inside an airship were simulated numerically. On the basis of the computational fluid dynamics method and user-defined function, external temperature boundary condition was imported into the wall grids. Under conditions of different steady states, the motion characteristics and influence regulations of gas natural convection inside the airship were analyzed by researching the characteristic parameters of flow field, such as distribution of pressure, temperature, speed. Besides, the unstable change process of natural convection was also studied preliminarily. The simulation results show that natural convection of helium inside a near space airship has some active influence on heat exchange of inside helium, but the influence is very weak on stressed skin and structural safety.