The tendency of reheat cracks of heat-resistant steel joints involved in the slit type cracking test was discussed. The results show that the cracks which start at the stress concentration at the root of the joint,propagate along the grain boundary of HAZ overheated zone and stop at the weld or HAZ fine grain zone are found in the slit type cracking specimen of BHW35 steel after stress relief heat treatment,and their properties are reheat cracks. The microstructures of HAZ coarse-grained zone are the necessary conditions for the formation of reheat cracks,while the restraint stress,hydrogen and high temperature behavior in the joint are the sufficient conditions for the formation of reheat cracks. In order to reduce the stress level in the joint,minimize the hydrogen content in the joint,control the formation conditions of reheat cracks,the comprehensive processes such as low stress and low hydrogenation and special welding materials are commonly used in production,and more satisfactory results have been obtained.