MoSe2　and　Bi2Se3　are　two　kinds　of　2D　materials　that　are　gradually　receiving　more　attention　because　of　their　unique　electronic　and　optical　properties.　Herein,　a　Bi2Se3/MoSe2　van　der　Waals　heterojunction　(vdWH)　is　constructed　and　the　electronic　and　optical　properties　of　the　heterojunction　are　calculated　using　first-principles　calculations.　The　effects　of　the　external　electric　field　and　the　interlayer　distance　on　the　electronic　properties　of　the　heterojunction　are　also　studied.　The　calculated　results　show　that　the　inherent　Bi2Se3/MoSe2　vdWH　has　a　type-2　band　alignment　with　a　very　small　indirect　bandgap　(28　meV).　It　also　has　strong　spin-orbit　coupling　effects,　and　the　characteristics　of　Zeeman　splitting　and　Rashba　splitting　are　observed　at　the　same　time.　Both　applying　an　external　electric　field　and　changing　the　interlayer　distance　can　effectively　modulate　the　band　structure　of　the　heterojunction;　these　modulation　methods　can　change　the　band　alignment　of　the　vdWH　from　type-2　to　type-3　or　even　type-1,　and　the　bandgap　type　can　be　changed　from　indirect　to　direct.　The　infrared　absorption　of　the　heterojunction　is　much　higher　than　that　of　Bi2Se3　and　MoSe2.　All　the　calculation　results　show　that　the　Bi2Se3/MoSe2　vdWH　has　good　application　prospects　in　optoelectronic　and　spintronic　devices.