Abstract: The 304 austenitic stainless steel guardrail on the balcony of a teaching building broke down.Macroscopic examination, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis were used to analyze the causes of the guardrail fracture.The results show that there were a large number of nonmetallic inclusions in the microstructure of 304 austenitic stainless steel guardrail base metal.The non-penetration of welded joint reduced the joint strength.The carbon content of base metal was higher,and under the action of welding thermal cycle,the carbon atoms with strong activity combined with chromium to form carbide precipitation,which made the grain boundary of heat affected zone chromiumdepleted and formed “sensitization”which increased the tendency of intergranular corrosion of materials,reduced the strength of welded joints,and ultimately led to the failure of guardrail.