With fundamental physical laws and fluid mechanics,a discharge fluid model of SF₆ in a short gap under uniform field is established.Flux-corrected transport(FCT) technique is used in numerical simulation of SF₆ discharge at atmospheric pressure. Dynamic characteristics of charged particles,distributions of gap-space electric field and evolutions of discharge channel are revealed. Tempo-spatial critical point of avalanche-to-streamer is discovered and shown.It shows that photoionization plays an important role in streamer development stage.