A charging method and a portable electronic device using the same are provided. The charging method includes following steps: detecting a battery voltage and a charging current of a battery module; determining whether the portable electronic device operates at a constant voltage (CV) charging mode; executing an impedance calculation at the CV charging mode to obtain a first battery voltage corresponding to a first predetermined current and a second battery voltage corresponding to a second predetermined current; calculating a compensation impedance according to the predetermined current and the battery voltages; setting a maximum charging voltage according to the compensation impedance and executing a CV charging to the battery module accordingly; determining whether a current variation of the charging current is larger than a threshold value; re-executing the impedance calculation; updating a setting value of the maximum charging voltage when the current variation is larger than the threshold value.