Combining CT disconnetion protection setting applying status, the paper explains requirement and pressing of technique capability efficacy. Exerting CT actual operating, designing trait and information, the paper introduces its basic functions, material circuit, test steps, judgement method of master parameter efficacy. It illuminates the scientificity of technique performance efficacy by practicing examples, hopes that the method brings into action in actual electric power system, and gains innovation & upgrade, promote the technique research and exploition of CT discon-nexion protection setting.