Insulating oil is the primary insulating medium of converter transformers, which may be subject to the influence of direct current superimposed with impulse voltage under working conditions. In this paper, a two-dimensional axisymmetric fluid simulation model of streamer discharge in mineral insulating oil is established based on the hydrodynamic drift-diffusion model. The simulation investigates the initiation and development of needle-plane streamer discharge in mineral insulating oil under short oil gap conditions. The characteristics of streamer discharge are analyzed in mineral insulating oil under various conditions, including different pulse, different voltage amplitude, and different direct current superimposed with impulse voltage conditions to provide theoretical and technical reference for the design of insulation of converter transformers.