Abstract: Analyzed in the paper are the reasons for surface cracking of the Mo alloy plug as occurring when it is being used to pierce the high Ni-Cr stainless steel. And an analysis is conducted via a comparison with the normal plug. As a result,it is identified that the said cracking of the Mo alloy plug is a kind of fatigue cracking,and the crack source area is located in the complex stress area on the surface of the piercing cone of the Mo alloy plug;since certain alloy elements,i.e.,Fe,Mn and Si are added to the cracked plug,the brittleness of the plug shall be increased by these elements directly or indirectly,and the oxides of Ti and Zr shall get the mechanical properties of the plug reduced;and under the condition of the hi-temperature complex stress,cracks first develop in the surface areas where holes or inclusions exist,and then the cracks continuously develop along the crystal boundaries,and further extend towards the inside of the plug.