The invention relates to a constant power charging system and a method of a lithium battery pack. A main controller is communicated with a sub-controller through a CAN bus, and after the sub-controller receives a command, a control voltage measuring module measures the voltage; a voltage detecting module transmits the detected voltage of a small battery pack to the sub-controller through a photoelectric isolation circuit, and the sub-controller transmits the information to the main controller; the main controller judges whether the voltage value is in the specified range or not according to the received measured total voltage V, wherein if the voltage value is in the specified range, the circulation continues, and if the voltage value is not in the specified range, the circulation stops; the main controller acquires the theoretical charging voltage by summing the total voltage V and the voltage loss delta V in a charging line; the main controller judges whether the Vtrue is less than the maximum charging voltage of a charger or not, wherein if the Vtrue is less than the maximum charging voltage, the circulation continues, and if the Vtrue is not less than the maximum charging voltage, the circulation stops; the main controller acquires the theoretical current value by dividing the theoretical charging voltage Vtrue by the maximum power of the charger; and the main controller transmits the Vtrue and the Vture to the charger through the CAN bus, and the charger charges a large battery pack according to the received voltage and current.