The invention provides a line voltage detection circuit and a related detection method. The embodiment provides a line voltage detection circuit, which is suitable for a power supply controller, comprising a high-voltage end which is connected to the line voltage through a high-voltage resistor. The line voltage is associated to AC input voltage. The line voltage detection circuit comprises a high-voltage starting transistor, a switchable current source, a bleeder circuit, and a management circuit. The high-voltage starting transistor is connected to the high-voltage end. The switchable current source is connected between the high-voltage starting transistor and a ground wire. The bleeder circuit comprises two divider resistors, which are in series connection between the high-voltage end and the ground wire, and is capable of providing a voltage-dividing result. Based on the voltage-dividing result, the management circuit switch can turn on/off the current source so as to optionally provide offset current to pass through the high-voltage starting transistor and the high-voltage resistor.