Pumpkin shaped balloon has been playing an important role in many space science explorations. This paper presents a computational study of buckling and post-buckling behavior of a 10 m diameter, 145 lobe pumpkin balloon, with lobes of approximately uniform radius. The ABAQUS commercial finite-element package is used. The study starts with a geometrically non-linear analysis to inflate the balloon modeled by linear-elastic membrane material which incorporates the wrinkling behavior. This is followed by an eigenvalue buckling analysis to predict the critical pressures corresponding to different buckling modes. Then, pristine new mesh models are set up with initial imperfection modes based on both the critical and combined eigenmodes. Finally, a geometrically non-linear post-buckling analysis is carried out to simulate the response of the balloon beyond the critical pressure.