This paper lists out some ordinary wrong connection ways of zero-order CT in lower voltage side of substation type transformer, and analyses the zero-sequence current’s direction and magnitude when the zero-order current fault.The serious results caused by wrong connection is proposed.Based on which, this puts forward the right connection way which is connected the ground connection and null line to the zero-order CT at the same time. And also it analyzes the fault situation to prove the stability of this connection way.