With regard to the requirement of the second type test eauipment to insulate the fault to SRU,this paper analyses the inside stucture of LRU and presents the concepts of point,rock-bottom test item(RBTI) and rock-bottom test item set(RBTIS).The correlative values are used to express the relationship between point and point,point and SRU,RBTIS and RBTIS.Then this paper puts forward the method of gradual elimination method for UUT with several SRUs.