Abstract：The increasing popularization and stable operation of IEC 61850 promote the demand ofintelligent upgrading traditional IEC 103 stations. There will be a long period of new and old systems running in parallel after starting substation transformation. During this period, there may be interlock information of BCU loss both in the new and old system. Also,monitoring five defense exists the same problem. On the basis of fully analyzing the needs of substation transformation, this paper draws on the existing schemes and excavate the essence of the problem, and puts forward the complete scheme of interlock agent and its accessing method. Compared with the existing agent, it has good characteristics of complete functions, complete scheme, simple configuration, stable and reliable and so on. The actual substation application proves the method is effective.