To　investigate　the　effects　of　fat　layer　on　the　temperature　distribution　during　microwave　atrial　fibrillation　catheter　ablation　in　the　conditions　of　different　ablation　time;　3D　finite　element　models　(fat　layer　and　no　fat　layer)　were　built,　and　temperature　distribution　was　obtained　based　on　coupled　electromagnetic-thermal　analysis　at　2.45GHz　and　30W　of　microwave　power.　Results　shown:　in　the　endocardial　ablation,　the　existence　of　the　fat　layer　did　not　affect　the　shape　of　the　50　degrees　C　contour　before　30s.　The　increase　speed　of　depth　became　quite　slowly　in　the　model　with　fat　layer　after　30s.　When　ablation　depth　needed　fixed,　there　are　no　significant　effect　on　effectively　ablation　depth　whether　fat　layer　over　or　not.　However,　the　existence　of　fat　layer　makes　the　temperature　lower　in　the　myocardium,　and　maximum　temperature　point　closer　to　the　myocardium　surface.　What　is　more,　in　the　model　with　fat　layer,　effective　ablation　reach　lower　maximum　temperature　and　the　shallower　depth　of　50　degrees　C　contour.　But　there　are　larger　ablation　axial　length　and　transverse　width.　In　this　case,　doctor　should　ensure　safety　of　normal　cardiac　tissue　around　the　target　tissue.　In　the　epicardial　ablation,　the　existence　of　fat　layer　seriously　affects　result　of　the　microwave　ablation.　The　epicardial　ablation　needs　more　heating　time　to　create　lesion.　But　epicardial　ablation　can　be　better　controlled　in　the　shape　of　effective　ablation　area　because　of　the　slowly　increase　of　target　variables　after　the　appearing　of　50　degrees　C　contour.　Doctor　can　choose　endocardial　or　epicardial　ablation　in　different　case　of　clinic　requirement.