Abstract: Pb-Bi cooled direct-contact-boiling water fast reactor (PBWFR) is of compact structure and portable characteristics, which is of great value to use in remote areas or islands. In this paper, the system analysis code SACOL and sub-channel analysis code SUBAS were coupled for analysis of the thermal-hydraulic characteristics of PBWFR. The results show that the relative errors of SACOL and coupled code do not exceed 4%, which proves the accuracy and rationality of the one-way coupling and step by step computation method. However, using SACOL for system analysis cannot obtain the detailed information of the core. Using coupled code can make up this shortage. By analyzing unprotected transient over power (UTOP) accident, it is shown that the cladding temperature increases fast and will exceed the allowable safety limit. Therefore, the cladding should be designed to have enough safety margins before the accident to ensure the safety operation in a long term under accident situation.