The present invention relates to a circuit minimizing standby power in a power adapter with a power-frequency transformer (T1), which includes a sensing circuitry (1), a driving circuitry (2), a switching circuitry (3) and a resistive element (Z). When the sensing circuitry (1) senses a power supply signal of the power-frequency transformer (T1) in work mode, the switching circuitry (3) shorts the resistive element (Z); and when the power-frequency transformer (T1) is in standby mode, the electronic switch (3) is switched off and the resistive element (Z) is coupled to the power-frequency transformer (T1). The circuit attached to the power adapter reduces the standby power consumption very much, and achieves energy-saving effect with low cost.