An isolated primary circuit regulator, using an average method for controlling an output current at a primary side of a transformer, the isolated primary circuit regulator comprising: a discharge time detector, configured to be connected to the transformer, for detecting a discharge time of a switching current generated at a secondary side of the transformer, and outputting a discharge time signal and an inverse discharge time signal; an oscillator, for generating an oscillation signal; a control circuit, comprising an output current waveform reconstruction circuit, for outputting an adjustment signal in response to the discharge time signal and the inverse discharge time signal by performing signal processing of output current waveform reconstruction by the output current waveform reconstruction circuit which outputs a square wave signal according to the discharge time signal, the inverse discharge time signal, a switching signal and a critical voltage, performing signal processing of filtering by a low-pass filter, amplification and comparison on the critical voltage, wherein the critical voltage is corresponding to a peak value of a switching voltage signal generated at the primary side of the transformer, the low-pass filter generating a filtering signal which is corresponding to the output current, and the square wave signal has an amplitude corresponding to the critical voltage, occurs in synchronization to the discharge time signal and has a waveform corresponding to a waveform of the output current; and a pulse width modulator, comprising a first input terminal and a second input terminal, for generating the switching signal, wherein the first input terminal is configured to receive the adjustment signal, the second input terminal is configured to receive the oscillation signal, the switching signal has a duty cycle and a frequency corresponding to the oscillation signal and the adjustment signal, and the control circuit is configured to regulate the output current of the transformer according to the adjustment signal. a discharge time detector, configured to be connected to the transformer, for detecting a discharge time of a switching current generated at a secondary side of the transformer, and outputting a discharge time signal and an inverse discharge time signal; an oscillator, for generating an oscillation signal; a control circuit, comprising an output current waveform reconstruction circuit, for outputting an adjustment signal in response to the discharge time signal and the inverse discharge time signal by performing signal processing of output current waveform reconstruction by the output current waveform reconstruction circuit which outputs a square wave signal according to the discharge time signal, the inverse discharge time signal, a switching signal and a critical voltage, performing signal processing of filtering by a low-pass filter, amplification and comparison on the critical voltage, wherein the critical voltage is corresponding to a peak value of a switching voltage signal generated at the primary side of the transformer, the low-pass filter generating a filtering signal which is corresponding to the output current, and the square wave signal has an amplitude corresponding to the critical voltage, occurs in synchronization to the discharge time signal and has a waveform corresponding to a waveform of the output current; and a pulse width modulator, comprising a first input terminal and a second input terminal, for generating the switching signal, wherein the first input terminal is configured to receive the adjustment signal, the second input terminal is configured to receive the oscillation signal, the switching signal has a duty cycle and a frequency corresponding to the oscillation signal and the adjustment signal, and the control circuit is configured to regulate the output current of the transformer according to the adjustment signal.