[en] The structure and operating procedure of the vacuum performance of in-vessel viewing system (IVVS) are introduced. The outgassing rate materials used for IVVS ware tested to evaluate if the system vacuum properties meet EAST vacuum environment requirements. The ultimate pressure provided by the tank for IVVS is 3.37 × 10^-3Pa under effective pumping speed of 1064L·s^-1. Analyses and optimization was made to improve the system vacuum properties, and finally the ultimate pressure is declined to a certain range of 1 × 10^-4Pa under the same pumping speed by replacing lower outgassing rate materials, bake-processing of IVVS and more optimized structure. After optimization, the vacuum performance of the system is obviously improved that can meet the requirements of vacuum environment accepted by EAST. Furthermore, the ultimate pressure can be still promoted by adding a set of cryopump to the vacuum pumping system in the tank. (authors)