The invention provides an RS 485 bus-based low-power control method and circuit, which are applicable to devices, of which the power supply part circuit in an inner machine and the power accessing part circuit in an outer machine are connected through an 485 bus. The power supply part circuit comprises a first relay, a first communication unit and a second communication unit, and the power accessing part circuit comprises a second relay, a third communication unit, a fourth communication unit and a main power supply wake-up unit. In this way, the problem when the inner machine and outer machine communicate with each other through an RS485 bus, the inner machine when being shut down or under a standby state can not cut off the power supply of the outer machine can be solved. The main power supply of the outer machine can be automatically cut off when the outer machine does not work, thereby saving the power consumption of a whole machine.