The utility model relates to an internal and external pressure balancing device for a logging instrument hydraulic system, which is arranged in the middle of a logging instrument, and comprises a first balance section outer pipe and a second balance section outer pipe that integrally join the shell of the logging instrument, as well as a first balance section inner pipe and a second balance section inner pipe that are arranged coaxial with the first balance section outer pipe and the second balance section outer pipe for electronic component cabling; a balance section seal sliding block is sleeved on the first balance section inner pipe, and the balance section seal sliding block divides the inner cavity of the first balance section outer pipe into a first hydraulic oil chamber and a second hydraulic oil chamber which are independent; a flexible leather bag used for filling system hydraulic oil is wrapped on the periphery of the second balance section inner pipe, and the outer wall of the second balance section outer pipe is provided with a liquid inlet communicated with external oil-water mixed liquor. According to the design, the balance section seal sliding block is arranged in a hydraulic circuit inside the logging instrument, dynamic seal of the hydraulic oil inside the flexible leather bag and the hydraulic oil in the hydraulic system is realized, the phenomenon that oil-water mixture enters the hydraulic system, caused by the breakage of the flexible leather bag, is completely eradicated, and the internal and external pressure balancing device provides guarantees for long-time and steady operation of the logging instrument.