For the structure of the secondary energy metering system, which is based on the analog-input MU in smart substation, a simulation system for the secondary energy metering in smart substation is builded, as well as its direction platform. Various factors which may cause the energy metering error are analyzed under the premise that the voltage MU, current MU (MU) and digitized electric energy meters meet the requirements of technical code. Then the key factors which cause the energy metering error and the influence level of other factors are identified after the impact test of the energy metering error. What"s more, the direction platform of on-site testing of secondary energy metering system in smart substation is proposed. As a result, the oversize of metering error is solved. #$NLKeywords: analog-input MU, secondary energy metering system, analysis of metering error, methods for on-site testing