The sampled value mapping model and realization method of the merging unit(MU) is analyzed. By using the synchronized sampling technique, the analog AC signals from the traditional voltage transformer and current transformer are constructed into a sampled value mapping in conformity with IEC 60044-7, IEC 60044-8 and IEC 61850-9-1 standards, directly laying the IEC 61850 data basis for traditional substations. The test method for MU is proposed and verified. Three sample units have successfully been developed with all technical parameters proved credible and accurate.