Abstract: The granular structure and granular bainite in the HAZ of 12Ni3MoV steel were investigated by means of weld thermal simulation method. The results obtained are as follows:The mirostructure in the critical region and the grain refined region is granular structure. The microstructure in the grain growth region is side-lath-granular bainite and acicular—granular bainite. The austeniti state significantly inluencs the transformation product during cooling. When the austenitic grain size is small othe austnitic component is inhomogeneous, granular structure will be formed during cooling. When the austenitic grain size is coarse and the austenitic component is homogeneous, granular bainite will be formed.