The invention relates to a polysilicon 
ingot casting method with low defect and high output and a thermal field structure thereof. The thermal field structure comprises a 
thermal insulation cover 
system, a 
crucible system, split resistance heaters, a support part and a cooling device, wherein the three heaters are independently and separably controlled to lower 
energy consumption to the maximum degree and bring convenience for regulating a 
crystal growth interface, thereby obtaining an evener 
crystal growth interface, improving 
crystal quality and solving the later-stage cooling problem of the 
crystal growth because of the increase of the height of the 
ingot casting. A support block with the unique design and a cooling device are combined to realize the purpose of local cooling, so that the local position on the bottom of a 
crucible generates cold spots at the early stage of the 
crystal growth; 
nucleation is carried out firstly to reduce 
nucleation spots, and lateral 
temperature gradient is obtained to provide the necessary condition for the lateral growth of the crystal 
nucleus; crystalline grains are enlarged to reduce a crystal boundary to the large extent; and microdefects, such as 
dislocation and the like, are lowered.