The new test material which was SA508-3 steel used for nuclear power has been subjected to tensile test in the six temperature levels(800℃～1300℃) by solidifying methods on Gleeble-1500D thermal simulation testing machine to analysis the microstructure changes. It was found that the strain, the reduction in area and the thermal strength were all the most value at 1000℃ temperature; the fracture organization of six temperatures were all different form lath martensite; the maximum hardness of 800℃, 1000℃ and 1300℃ test sample were at 10mm, 14mm and 18mm，respectively, the hardness range ability of 1000℃was the lesser, the two other were the larger extent; The fracture morphology of low temperature plastics was main inter granular brittle fracture, which provided the basic parameters has important practical value for weld numerical simulation.
