The invention relates to a reservoir with a charging flexible diaphragm volume compensation system and a use method. According to a main technical scheme, a flexible diaphragm device between a reservoir body and an air volume compensator does fluctuating movement of swelling upwards and sinking downwards elastically along with the volume change of liquid in the reservoir, resulting in the air volume change of the air volume compensator arranged at the upper part of the flexible diaphragm device of the reservoir, and the air volume change of the air volume compensator generates breathing communication action with outside atmosphere via an airflow exchange tube and a three-way bypass breathing solenoid valve, thereby balancing pressure and volume change caused by the level height change of the liquid in the reservoir and reaching the aim of sealing the liquid stored in the reservoir. According to the invention, the physical and chemical properties of the liquid can be kept unchanged for a long time, the liquid can be taken freely when used and added and compensated at any time, energy consumption is reduced greatly, work intensity is lowered, work time is shortened, environment pollution caused by escaping, overflowing, dripping and leaking can be alleviated greatly, and thus the reservoir has huge advantages.