In this paper, on the basis of analyzing the core work for train network control system SIL certification, the key contents of SIL assessment activity process in TCMS development safety lifecycle were summarized, and a Heatshrink specific method for the SIL index assessment was brought up.In particular, a clear schematic description on determination and allocation process Link Assembly for the safety requirements and tolerable risk failure rate of TCMS was conducted.Meanwhile, a calculation method for failure in time (FIT) of safety function was also described in detail.