A current detecting circuit of a constant on-time power converter and a method thereof are provided. The current detecting circuit includes a time detecting circuit and a sample and hold circuit. A control circuit of the constant on-time power converter outputs a lower bridge conduction signal to turn on a lower bridge switch during an on-time of each cycle of the lower bridge conduction signal. The time detecting circuit detects the on-time of a cycle of the lower bridge conduction signal. The sample and hold circuit samples and holds a current of an inductor at a detection time point of a next cycle of the lower bridge conduction signal. A time between a rising edge of a waveform of the next cycle of the lower bridge conduction signal and the detection time point is equal to half the on-time.