A multi-stage power converter, comprising: a first-stage power conversion circuit; a second-stage power conversion circuit comprising a transformer, a primary side circuit and a secondary side circuit, wherein the primary side circuit is connected between the first-stage power conversion circuit and a primary winding of the transformer, and the secondary side circuit is connected with a secondary winding of the transformer; a second-stage analogic sampler connected with the secondary side circuit for sampling at least one second-stage power parameter from the secondary side circuit; a pulse width modulator arranged to receive a sampling result from the second-stage analogic sampler and configured to convert the at least one second-stage power parameter into at least one pulse width modulation signal with at least one corresponding duty ratio; a first isolator connected with the pulse width modulator for receiving the at least one pulse width modulation signal; a pulse width analyzer connected with the first isolator, wherein the pulse width analyzer is arranged to receive the at least one pulse width modulation signal through the first isolator in an isolation manner and is configured to calculate the duty ratio of the at least one pulse width modulation signal according to a rising edge and a falling edge of the at least one pulse width modulation signal and to calculate the at least one second-stage power parameter according to the duty ratio; and a control unit connected with the first-stage power conversion circuit, the primary side circuit and the pulse width analyzer, wherein the control unit is configured to control operations of the first-stage power conversion circuit, and to control operations of the primary side circuit according to the at least one second-stage power parameter that is obtained by the pulse width analyzer, characterized in that the at least one second-stage power parameter sampled by the second-stage analogic sampler includes plural second-stage power parameters, and the pulse width modulator is configured to generate plural pulse width modulation signals with different frequencies in a time division manner, wherein the first isolator comprises a single isolation channel, and pulse width modulator is configured to transmit the pulse width modulation signals to the isolation channel in a time division manner, wherein the plural pulse width modulation signals have fixed or variable cycle numbers. a first-stage power conversion circuit; a second-stage power conversion circuit comprising a transformer, a primary side circuit and a secondary side circuit, wherein the primary side circuit is connected between the first-stage power conversion circuit and a primary winding of the transformer, and the secondary side circuit is connected with a secondary winding of the transformer; a second-stage analogic sampler connected with the secondary side circuit for sampling at least one second-stage power parameter from the secondary side circuit; a pulse width modulator arranged to receive a sampling result from the second-stage analogic sampler and configured to convert the at least one second-stage power parameter into at least one pulse width modulation signal with at least one corresponding duty ratio; a first isolator connected with the pulse width modulator for receiving the at least one pulse width modulation signal; a pulse width analyzer connected with the first isolator, wherein the pulse width analyzer is arranged to receive the at least one pulse width modulation signal through the first isolator in an isolation manner and is configured to calculate the duty ratio of the at least one pulse width modulation signal according to a rising edge and a falling edge of the at least one pulse width modulation signal and to calculate the at least one second-stage power parameter according to the duty ratio; and a control unit connected with the first-stage power conversion circuit, the primary side circuit and the pulse width analyzer, wherein the control unit is configured to control operations of the first-stage power conversion circuit, and to control operations of the primary side circuit according to the at least one second-stage power parameter that is obtained by the pulse width analyzer, characterized in that the at least one second-stage power parameter sampled by the second-stage analogic sampler includes plural second-stage power parameters, and the pulse width modulator is configured to generate plural pulse width modulation signals with different frequencies in a time division manner, wherein the first isolator comprises a single isolation channel, and pulse width modulator is configured to transmit the pulse width modulation signals to the isolation channel in a time division manner, wherein the plural pulse width modulation signals have fixed or variable cycle numbers.