Abstract: In ultra high vacuum environment, Bayard-Alpert ionization vacuum gauges generate ion current by ionizing gas molecules.This current signal is very weak and susceptible to interference.This paper introduces a weak signal detection system for ultra-high vacuum measurement, which can meet the wide range current signal detection from mA to subpA.It is composed of measurement module, zero adjustment module and processing module.High precision amplifier and highly insulating relay are selected to realize the precision amplification of weak signals. The system can automatically complete the measurement gear change, zero adjustment, data processing and pressure calculation. The lower limit of measurement up to 10^-13A with good measurement linearity.It has been applied for the accurate measurement of ion flow of ultra high vacuum gauges, showing good measurement stability.