A power adapter for a portable computer includes a pulse width modulation unit and an output shaping unit and further includes a first judgment module that judges whether the portable computer is in a first state according to state judging parameters, and compares an output voltage with a first preset voltage and a second preset voltage when judging that the portable computer is in the first state, and acquires a judging result, wherein the first preset voltage is greater than or equal to the second preset voltage. The power adapter can increase efficiency.