The metal-dielectric-vacuum junction is defined as the triple junction owned enhanced electric field,thus this special region is regarded as the location where primary electrons emission is favored.For electron emission,triple junction could affect both the flashover breakdown of insulators and the electron emission property of ferroelectric cathodes.In this study,we theoretically investigate the electric field enhancement in the triple-junction region.It is found that the key parameter to determine the field enhancement is the taper angle of the electrode and the relative permittivity of the dielectric.In addition,we first deduce the accurate expression of the electric field in this special region.The controlling parameters for determining the field enhancement are discussed in detail.We also discover the way to reduce the electric field of this region through simulation.The current analysis would be useful for both the electron emission enhancement and the issue of flashover breakdown.