Abstract: An Ethernet data transmission circuit, an Ethernet data transmission system and an Ethernet data transmission method are provided. The Ethernet data transmission circuit includes: a polarity processing circuit for processing a polarity carried by Ethernet data into a preset polarity; and an encoder for receiving the Ethernet data and the preset polarity carried by the Ethernet data, and encoding the Ethernet data. On the one hand, the security of Ethernet in a transmission process can be improved; on the other hand, without increasing workload of the encoder, the polarity processing circuit of the Ethernet data transmission circuit can be used to process the Ethernet data to be with a preset polarity, to facilitate the encoder to code.