IEC60870-5-104 Protocol is widely used in Substation Subsystem. Paper briefly analyzed the 104 Protocol. Aimed at present problems about interconnection between different manufacturers" equipments, the paper proposes the simulation testing methodology of Substation Subsystem based on IEC104 Protocol. Then paper introduces the test system design including Conformance Test and Function Test. It is contributes to the interconnection of whole subsystem. It has a positive significance for Smart Substation construction and stable operation. According to the test results, it is proved that the testing system which hasSgoodSapplication value is validity and reliability.