This paper presents a scheme of high speed data communication system based on medium voltage (10 kv) power- line for power distribution automation. A novel substation roaming algorithm aiming at network reconstruction and signal automatic relay transmission is proposed. In the development of communication terminal equipment, the high speed data communication technique used in low voltage power-line communication systems is upgraded to medium voltage systems by utilizing a specially designed high frequency broadband power amplifier to linearly amplify OFDM (orthogonal frequency division multiplexing) signal flow in order to overcome long distance attenuation which is more extrusive in medium voltage distribution network. The effective injection of OFDM high speed data into medium voltage power-line has been realized. It can satisfy about 10 km transmission requirement.