Abstract: In order to detect the extremely low ion current in ionization gauge when it is used to measure extremely high vacuum,a current-to-voltage conversion circuit equipped with low-pass filter,zero-point adjustment and phase compensation technology is designed in this paper,which can accurately measure the extremely low ion current about 10^-14 A. By verifying present circuit with different gauge type,it is found that the designed circuit can precisely detected the extremely low ion current generated in 10^-9 Pa.This circuit provides a technical reference for achieving extremely high vacuum equipment localization.