The　theoretical　and　experimental　investigations　of　the　depolarization　of　cylindrical　vector　beams　when　they　propagate　into　a　side　non-uniformly　pumped　isotropic　laser　medium.　The　azimuthal　gradient　of　thermal　load　inside　the　cross-section　of　laser　rod　and　its　induced　dynamic　process　on　thermally　induced　birefringence　are　the　internal　mechanism　of　the　depolarization　of　the　cylindrical　vector　beams.　The　experiment　results　show　that　the　depolarization　degree　is　proportional　to　the　azimuthal　gradient.　For　the　experiment　level　of　depolarization,　the　maximum　degrees　of　both　radially　and　azimuthally　polarized　beams　measured　at　the　azimuthal　gradient　of　0.35　W/(mm(3).rad)　were　3.1%.