In　current　study,　microstructure　and　texture　evolutions,　the　mechanism　of　variant　selection　for　{10-12}　twins,　the　nucleation　and　propagation　of　twins　during　uniaxial　multi-pass　compression　in　an　extruded　AZ31　Mg　alloy　were　investigated　by　in-situ　EBSD　technology.　The　specimen　was　compressed　along　ND　to　different　strains　of　1.9%,　3.4%　and　6%　at　a　strain　rate　of　1　mm/min　at　room　temperature.　The　results　show　that　the　yield　stress　maintains　increased　during　the　uniaxial　multi-pass　compression.　Due　to　the　growth　and　propagation　of　the　pre-generated　twins,　the　ultimate　stress　of　last　compression　is　approximately　equal　to　the　yield　stress　of　next　recompression　along　ND.　It　is　found　that　the　mechanism　of　variant　selection　for　{10-12}　tension　twinning　is　dominated　by　both　high　Schmid　factor　(SF)　and　high　strain　compatibility　factor　m＇　in　extruded　Mg　alloys.　Meanwhile,　the　strain　compatibility　plays　an　important　role　in　the　propagation　of　twins　in　common　grain　boundaries.　Two　types　of　grains　were　discussed　in　the　paper:　grains　containing　one　twin　variant　(one-twin　grains)　and　grains　containing　two　twin　variants　(two-twin　grains).　In　the　one-twin　grains,　the　twinning　growth　speed　shows　scattered　distribution　after　three-pass　compression.　Twin　growth　and　propagation　are　affected　by　the　maximum　length　of　activated　twins.　In　addition,　the　misorientation　of　twinning　boundaries　can　affect　the　expanding　of　pre-generated　twins.　While　in　the　two-twin　grains,　the　variants　with　para-position　(PP)　relationships　(7.4　degrees　＜　1-210　＞)　can　promote　the　twin　growth　and　coalescence.　More　nucleation　sites　and　close　mis-orientations　between　two　variants　lead　to　the　high　speed　of　twinning　growth　and　propagation　in　those　two-twin　grains.　This　is　the　reason　why　two-twin　grains　can　achieve　higher　volume　fraction　of　twins　compared　with　that　in　one-twin　grains.