The invention discloses a protocol conversion method of a relay protection device application gateway on a 10kV line. The protocol conversion method comprises the following steps: 1, configuring a protocol and equipment related information; 2, receiving data, and identifying a complete data frame; 3, identifying a protocol to which the data frame belongs and the type of the data frame; 4, matchingthe data frame with a corresponding protocol set, and if matching succeeds, reading a data value of the data frame and multiplying the data value by a coefficient to calculate a parameter actual value; 5, querying a protocol conversion set, and finding a target protocol number and a target data frame number; 6, searching for data items in the target protocol set, and converting the actual parameter value into a target protocol transmission value; 7, after conversion is finished, constructing a target data frame; and 8, sending the target data frame to an electric power SCADA or a corresponding relay protection device. According to the method, the flexibility and the expandability of the application gateway are improved.