Due to the adoption of Ethernet architecture，new requirement of information security is presented for smart substation based on IEC61850standard. Manufacturing message specification（MMS），sample measured value（SMV）and generic object oriented substation event（GOOSE），are common messages used in smart substations，whether these messages can be transmitted secretly is extremely important for the security of entire substation system. If each message uses a mode of authentication encryption algorithm，that may increase the burden of network equipments and affect their inter-operability. A message security mechanism and the message format for the secure transmission of smart substation messages based on GCM（Galois/counter mode）are designed. Taking GOOSE message as an example，the secure message transmission algorithm design，and its time cost based on GCM are analyzed. Finally，advantage analysis of secure message transmission method based on GCM for smart substation is carried. With the analysis，it is concluded that the common message authentication encryption algorithm based on GCM is adopted for different security requirements of messages and secure transmission of different messages is achieved through choosing different algorithm modes. That can improve security of smart substation information and network performance.