The invention relates to the field of cracking of waste polyolefin, and concretely relates to an apparatus and a method thereof for directly heating and cracking waste polyolefin by an oxy-hydrogen combustion method. The apparatus comprises an oxy-hydrogen combustor (1-3) and a pyrolysis reaction apparatus (1-1), the oxy-hydrogen combustor (1-3) is communicated to the pyrolysis reaction apparatus (1-1), the pyrolysis reaction apparatus (1-1) comprises a pyrolysis reactor (2-1), the pyrolysis reactor (2-1) comprises a mixer (2-1a) and a reactor (2-1b) which are connected up and down; an overheated steam inlet (3-1) and a gas-liquid mixing material inlet (3-3) are respectively arranged on two opposite sidewalls of the mixer (2-1a) along the tangential direction, and a gaseous state material inlet (3-2) is arranged at top of the mixer (2-1a) along the axial direction. The apparatus uses a tangential or axial mixing mode of overheated steam and the material to effectively avoid over cracking or incomplete cracking due to uneven mixing.