A　simplified　mechanical　model　for　investigating　pile-pile　horizontal　vibration　is　developed　by　combining　the　Pasternak　model　and　the　Timoshenko　beam　theory,　with　the　consideration　of　the　influence　of　axial　load　at　the　head　of　the　source　pile.　The　expressions　for　horizontal　displacements　and　internal　forces　of　the　source　pile　are　obtained　with　respect　to　the　differential　transform　method　and　pile-soil　boundary　continuity　conditions.　Additionally,　considering　that　the　vibration　of　the　source　pile　affects　the　dynamic　displacements　of　the　receiver　pile,　the　dynamic　equilibrium　equation　of　the　receiver　pile　is　established.　Then,　a　closed-formed　solution　for　the　response　of　the　receiver　pile　is　derived　from　the　obtained　expressions.　Further,　the　pile-pile　horizontal　dynamic　interaction　factor　is　derived　based　on　its　definition　and　validated　by　comparison　with　existing　related　analytical　solutions.　Finally,　the　effects　of　the　shear　deformations　of　soil　and　pile,　pile　slenderness　ratio,　and　axial　load　on　the　pile-pile　horizontal　dynamic　interaction　factors　are　examined.　It　is　indicated　that　a　smaller　pile-to-soil　modulus　ratio　of　the　near-surface　layer　results　in　a　more　significant　effect　of　shear　deformation　of　the　soil.　In　particular,　when　L/d　is　small　(i.e.,　L/d　＜　5)　or　the　pile　cross　section　is　annular,　the　effect　of　shear　deformation　of　the　pile　is　not　ignorable.