For ballonets with various shapes at 0~20 km, the incompressible Navier-Stokes(N-S) equations were solved by artificial compressibility approach, and steady and unsteady flow fields around ballonets at five different altitudes were numerically simulated. The ballonet aerodynamic force and the complex flow separation at leeward region with changes of altitude and its shape for the steady condition were simulated. The results show that the complex spatial flow separation exists at leeward region when the ballonet thickness rate increases gradually. The unsteady flow field numerical simulation at translational or pitching motion conditions was performed successfully by applying dual-time method and dynamic grid technology. The effects of translational direction and velocity on ballonet unsteady characteristics were investigated. The variations of aerodynamic force and flow field during the translational process were obtained. For the pitching motion, the ballonet aerodynamic force and flow field at various pitching speeds and maximum angles were achieved. The calculation results show that the ballonet unsteady motion has evident effects on its flow field and aerodynamic force.