Abstract: The safety assurance work of urban rail transit FAO (fully automatic operation) system includes: to define the safety-related system scope and functions according to factors such as system functionality, application scenario and fault impact; to carry out systematic analysis of the safety functions of the safety-related system and to assign and determine the SIL (safety integrity level). Taking the ISCS (integrated supervisory control system) of certain rail transit line FAO system as an example, the sufficient, reasonable and feasible safety assurance work are discussed, and the safety assurance mode of the safety-related system to meet the safety function SIL requirements is studied.