A mammographic X-ray system, a compression device and a pressure detection device (2) thereof: the pressure detection device (2) is connected between a drive device (1) of the compression device and a compression plate (3); the pressure detection device (2) comprises a support frame (10) which is mounted on the drive device (1), a conduction plate (20) on which the compression plate (3) is mounted, a pressure sensor (30) which is communicatively connected to a control device of the mammographic X-ray system, and a shaft assembly (40); the pressure sensor (30) is arranged on the support frame (10), while the conduction plate (20) is connected to a side of the support frame (10) by means of the shaft assembly (40) and may rotate relative to the support frame (10), being in detachable contact with the pressure sensor (30). The pressure detection device (2) of the mammographic X-ray system compression device has a simple structure and sensitive pressure response, which greatly reduces pressure errors when acting on the human breast.