The invention relates to a pressure balancing device in a petroleum drilling downhole instrument or tool in the field of downhole operation tools. The technical scheme is that a wiring pipe is arranged in a hollow joint, the upper end and the lower end of an annular cavity of the wiring pipe and the joint are respectively plugged by an oil passage plug and a plug, a floating piston together with a spring is sheathed outside the wiring pipe between the oil passage plug and the plug, one or a plurality of oil passages are axially arranged in the oil passage plug, the inner wall and the outer wall of the floating piston are respectively in sliding seal fit with the wiring pipe and the joint, and one or a plurality of liquid passages are radially arranged in the annular cavity section of the middle part of the joint. The pressure balancing device has the advantages that the required hydraulic oil can be provided for the rotating seal of the downhole instrument or tool, the reliability and the service life of a sealing system are improved, and the downhole normal operation of the downhole instrument or tool is guaranteed.