The present invention provides a tightness detection device which is formed by a detection mechanism and a control system. By using the 'pressure drop method' detection principle, a detected iron box is placed in an openable sealed box, then the sealed box is closed, an interlayer space is pumped to be vacuum, a pressure difference with the inner cavity of the iron box, and the tightness of the iron box is evaluated through measuring the interlayer space pressure change value in a predetermined time. Through adding a comparison tank with the approximate volume of the interlayer space, a detection process is divided into two steps, the first step is forming pressure balance in the comparison tank and the interlayer space and using a balanced pressure value to judge whether a large flow leakage condition exists, and the second step is using the 'pressure drop method' to detection a small leakage condition. The tightness detection device provided by the invention is used in a bullet automatic package production line, the online lossless detection of the tightness of the iron box can be realized, and the tightness detection of a national standard iron box and an American standard iron box can be realized through replacing a mold.