Abstract: Ion-trap is a device usually used in spectroscopic studies and it only works in cryogenic ultra high vacuum environment.A set of cryogenic ultra high vacuum system of ion-trap is introduced in this paper.The system is composed of vacuum chamber,vacuum pumping system,temperature monitoring and control system,pulse tube refrigerator,etc.This paper analyzes the key factors which affect ultimate pressure of the vacuum system through vacuum-pumping comparative tests under three different conditions.Adopting ultra high vacuum obtaining method and technics,the vacuum system obtains vacuum degree of 1.9×10^-8 Pa and 5.0×10^-10 Pa under room temperature and cryogenic environment separately.And the minimum temperature of ion trap reaches 3.9 K under high vacuum adiabatic conditions.The development process as well as the results of the system is summarized at the end of the paper and the design and development method which is applicable to similar cryogenic ultra high vacuum system is proposed also.