Abstract: Communication based train control system (CBTC) are widely used in Urban Transit. Because of its advantages on train locating and train tracking, the implementation of the interlocking function has been improved. Traditional setting of approach section was unable to meet the requirements of Urban Transit, however, the relevant specifications for approach section of Urban Transit are not given. In this paper, according to the characteristics of the CBTC mode and backup mode, the influence factors for the settings of approach sections were analyzed, the lengths and release delay time of approach section were calculated. In addition, actual situations were discussed and analyzed.