Disclosed is a vacuum container, its cap and a vacuum container assembly. The vacuum container assembly comprises a vacuum container and an air extraction device. The vacuum container comprises a vessel and a cap, which jointly define a containment space. The vessel has an upper edge, and the cap comprises a first portion, a second portion, a third portion, and an air valve. The first portion accommodates the upper edge of the vessel. The second portion can be deformed under an external force and be restored to its original shape upon removal of the external force. The third portion and the first portion jointly surround the second portion, and the third portion is equipped with an air valve. The air extraction device comprises an opening, which is fitted with the third portion of the cap. The ratio between the thickness of the third portion adjacent to the second portion T2 and the thickness of the second portion of the cap adjacent to the third portion T1 (T2/T1) is at least equal to or greater than 1.1. Air inside the containment space is evacuated through the third portion by exerting an external force on the second portion and/or by driving the air valve with the air extraction device.