The electrooxidation of propylene into propylene oxide under ambient conditions represents an attractive approach toward propylene oxide. However, this process suffers from a low yield rate over reported electrocatalysts. In this work, we develop an efficient electrocatalyst of Ag<sub>3</sub>PO<sub>4</sub> for the electrooxidation of propylene into propylene oxide. The Ag<sub>3</sub>PO<sub>4</sub> cubes with (100) facets exhibit the highest yield rate of 5.3 g<sub>PO</sub> m<sup>-2</sup> h<sup>-1</sup> at 2.4 V versus reversible hydrogen electrode, which is 1.6 and 2.5 times higher than those over Ag<sub>3</sub>PO<sub>4</sub> rhombic dodecahedra with (110) facets and tetrahedra with (111) facets, respectively. The theoretical calculations reveal that the largest polarization of propylene on Ag<sub>3</sub>PO<sub>4</sub> (100) facets is beneficial to break the symmetric π bonding and facilitate the formation of C-O bond. Meanwhile, Ag<sub>3</sub>PO<sub>4</sub>(100) facets exhibit the lowest adsorption energies of <sup>*</sup>C<sub>3</sub>H<sub>6</sub> and <sup>*</sup>OH, inducing the lowest energy barrier of the rate-determining step and thus accounting for the highest catalytic performance.