Abstract: Accelerator-driven sub-critical system （ADS） project is one of “973 Project”, which aims to solve ADS key issue. The design of principle verification facility could provide the platform of solving ADS key issue and engineering design. Based on two different core schemes, the paper presents the thermal-hydraulic theoretical analysis on ADS principle verification device. It consists of initial steady and transient accidental analysis. To find out the level of natural circulation, loss of flow was researched in transient analysis. In the end, the seriousness of transient cases and the result of two schemes were compared. The results show that in transient accident the natural circulation can remove core residual heat enough; and the first scheme is bigger than the second scheme in terms of safe margin.