The invention discloses a lithium ion battery DC power supply system floating charge operation control method and a lithium ion battery DC power supply system floating charge operation control system. The method is characterized by comprising steps: the lithium ion battery is charged; at the stage of subsequent resting after charging, the output voltage of a charging device is controlled; at the subsequent floating charge stage, the output voltage of the charging device is controlled; and after the floating charge stage, power is supplemented to the lithium ion battery. The system comprises a control device, a charging device, a lithium battery current sensor and a lithium battery module string. The method and the system have the beneficial effects that at the stage of resting after charging, the output voltage of the charging device is equal to open circuit voltage of the lithium ion battery, the lithium ion battery does not perform discharging and a full capacity is maintained; at the floating charge stage, the output voltage of the charging device is equal to the open circuit voltage after expected power supplementation for the lithium ion battery and resting, power loss caused by self discharging of the lithium ion battery is supplemented, and over charging is avoided; and through supplementing the power, insufficient power of the lithium ion battery caused by long-time floating charge can be avoided.