At present there is few researches on shed parameters of transformer bushing at home and abroad. In addition, most of the existing studies on flashover characteristics of the bushing use hollow bushing as experiment object without considering the influence of the internal conductive tube on the insulation electric field. In this paper, a 500 kV transformer high-voltage bushing equivalent model is established by finite element software COMSOL in order to study the effect of shed spacing and overhang on bushing external electric field under rain condition. Meanwhile simplified model without conductive tube and capacitance core is established to study the effect of conductive tube and condenser on bushing external electric field. The results show that the increase of shed spacing leads to obvious decline of air gap field relative range, and also leads to subtle increase of air gap field strength. The average electric field along the surface path decreases linearly with the increasing of shed overhang, and the relative range decreases with the increasing of shed overhang when big shed overhang is in range of 85~100 mm. Without conductive tube, the electric field intensity increases slightly at the shed depressions while decreases at the shed edges. And bushing external electric field is similar to the equivalent model with condutive tube.