A charger calibrating device and a calibrating method thereof. The device comprises a control module and a processing module. The control module controls a charger to be calibrated to perform a first stage charging and a second stage charging on an electronic device. The processing module performs an adjusting process according to the second stage charging time for adjusting the high level period of the PWM signal in the charging circuit of the charger. In the adjusting process, generating an updated high level period by adding or decreasing a preset adjusting amplitude, and decrease the preset adjusting amplitude by half to generate an updated adjusting amplitude. The processing module terminates the calibrating process after repeating the aforementioned calibrating loop a preset number of times.