Abstract: Analyzed here in the article are the causes for the surface network-like cracks as staying with the Φ610 mm×50 mm WB36 steel tube,and studied are the results of observation of the macro-morphology and micro structure,the SEM analysis and energy spectrum test of the said cracks. It is regarded via the studies that the cracks of the said steel pipe are originated from the crack of the billet. In case of over high casting temperature,the austenite grain size of the steel is enlarged,and moreover the enrichment of the chemical element Cu as contained in the steel is developed during the solidification process,both of which act as the main causes for the above mentioned network-like surface cracks of the pipe.