This invention relates to a mobile electronic device and a power management method of a battery module thereof. The mobile electronic device includes a battery module, a charging/discharging module, and a control module. The charging/discharging module is coupled with the battery module. The control module is coupled with the battery module and the charging/discharging module. When the power supply is coupled with the mobile electronic device and the system time is not within a maintenance period, the control module controls the charging/discharging module to maintain capacity of the battery module within a first capacity range. When the power supply is coupled with the mobile electronic device and the system time is within the maintenance period, the control module controls the charging/discharging module to maintain the capacity of the battery module within a second capacity range.