A series of solid electrolytes KNb1-xMgxO3-δ (x=0.0—0.3) with an orthorhombic perovskite structure was synthesized under high temperature and high pressure（4.0GPa，870℃）. The effect of Mg-doping on the structure transformation and electrical properties was systematically studied in this paper. The Raman spectra at different temperatures and differential thermal analysis results indicate that the structure of the samples transforms from ferroelectric orthorhombic to tetragonal phase, and finally to paraelectric cubic phase with increasing temperature. Weakening of the coupling effect of A-site cation on BO6 octahedra and the B-site cation contribution to spontaneous polarization, as a result of the Mg2+ doping in KNbO3, leads to th e decrease of Curie temperature.The Curie point of KNb0.85Mg0.15 O2.775,decreasing by about 40℃ in comparison with KNbO3 ,is 392℃.Impedance spectroscopic data show that the conductivity of the samples is ionic, the grai n-boundary effect is very significant, and the conductivity of the samples is de termined mainly by the grain-boundary. Mg-doping increases the oxygen ion conduc tivity of the samples. KNb0.9Mg0.1O2.85 has the highest conduc tivity of 1.2×10-3S/cm at 700℃ in all of the samples,which makes it a kind of promising oxygen ionic conduction materials.