The invention provides a device for preventing looseness of a cap for a cross head and a piston rod of a compressor. The device comprises the cross head (1). A connecting part (2) is arranged at one end of the cross head (1). The cap (3a) is arranged on the piston rod (3). A clamping block (4) correspondingly matched with the cap (3a) and the connecting part (2) is arranged between the cap (3a) and the connecting part (2). The device is characterized in that a circular lug is arranged on the outer wall of the cap (3a), and lug sawteeth (3b) are arranged on the outer circumferential surface of the lug; and a pressing plate (5) is arranged, pressing plate sawteeth (5a) correspondingly matched with the lug sawteeth (3b) are arranged on one side of the pressing plate (5), and the pressing plate (5) and the clamping block (4) are connected through a screw (4a). The device is simple in structure and convenient to use and can be directly transformed on existing equipment, and the situation that the cap gets loose is effectively avoided.