A multi-stage power converter includes a first-stage power conversion circuit, a second-stage power conversion circuit, a second-stage analogic sampler, a pulse width modulator, a first isolator, a pulse width analyzer and a control unit. The second-stage analogic sampler samples power parameter from the second-stage power conversion circuit. The pulse width modulator converts the power parameter into a pulse width modulation signal. The pulse width analyzer receives the pulse width modulation signal through the first isolator in an isolation manner, calculates a duty ratio of the pulse width modulation signal according to a rising edge and a falling edge of the pulse width modulation signal, and calculates the power parameter according to the duty ratio. The control unit controls operations of the second-stage power conversion circuit according to the power parameter that is obtained by the pulse width analyzer.