Twin types in a room-temperature compressed magnesium alloy (Mg-3Al-1Zn) sample were identified by using electron backscattered diffraction (EBSD) technique, and the results indicate that most of the twins are {10\\bar12} twins and only a few of them are {10\\bar11} twins. In order to study the law of atomic motion in the {10\\bar11} twinning, we calculated the displacement vectors of the twinning atoms in the {10\\bar11} twinning and found that the atomic motion can be explained through a model named quadrangular prism-shaped atomic group (QPAG). In the QPAG model there exist two types of alternately distributed QPAG units totally. Though the rotational angle of the two types of QPAG units in the {10\\bar11} twinning is smaller than in the {10\\bar12} twinning, the relative displacement magnitude in the {10\\bar11} twinning is larger than in the {10\\bar12} twinning due to its more complicated atomic motion, and this should be the reason that the {10\\bar11} twinning is harder to occur than the {10\\bar12} twinning.