Electrolytic Li3PO4 coating on Pt as the solid electrolyte has been carried out in the mixture of LiNO3 and NH4H2PO4 aqueous solution. The cathodic reactions during the polarization could be divided into three stages: (I) O2 +2H+ +2e− →2OH− (+0.49 to −0.33V); (II) 2H+ +2e− →H2 (−0.33 to −0.68V); (III) 2H2O+2e− →H2 +2OH−, HPO4 2− +OH− →PO4 3− +H2O and H2PO4 − +2e− →PO4 3− +H2 (−0.68 to −2.0V). The orthorhombic β-phase Li3PO4 was found until the transition to orthorhombic γ-phase at 500°C. The lithium ionic conductivity of 100nm crystalline Li3PO4 thin film was about 8.62×10−8 Scm−1 at 25°C. Also, the mechanism of electrolytic Li3PO4 coating on Pt was discussed in this article.