The paper reports on the atomic investigation about β phase in Mg₉₆Gd₂Y₁Ni₁ alloy by using the first-principles study and the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) corrected by atomic Cs. By using HAADF-STEM, the rectangular β phases were observed in the underage and peak aging stages in Mg₉₆Gd₂Y₁Ni₁ alloy. The β phase could be precipitated from the previously precipitated β phase, and the β phase grew in steps when it was precipitated. A special transition structure of three atomic layer thicknesses was first observed at the edge of the β phase and the structure of this interface is probably as the β/Mg₁ interface for the analysis of thermodynamic characterization and electronic characterization. The β' phase and the β_H structure were precipitated only at the edge of the length directions of the β phase. The β' phase continues to grow into a β phase directly without the formation of β₁ phase, resulting in an increase in the length of the β phase, which is discovered for the first time.