The present disclosure provides a serial port communication mode conversion method, system, and circuit for a serial port circuit, in which the serial port circuit includes a first serial port and a second serial port, a transmission signal line of the first serial port and a reception signal line of the second serial port are connected to form a half-duplex signal line. The method includes: configuring the second serial port to enable after a preset time; controlling the first serial port to transmit a control signal to an external device through the half-duplex signal line within the preset time; and controlling the second serial port to receive response data transmitted by the external device through the half-duplex signal line after the preset time. The present disclosure realizes the conversion of the full-duplex serial port to the half-duplex serial port.