The traffic alert and collision avoidance system( TCAS) is an important part of the surveillance system and plays an important role in avoiding dangerous approach and ensuring safety. In view of the difficulties of TCAS failure in dispatch, the analysis was carried out from three stages: before taxi out with its own power, after taxi out with its own power, and after takeoff. First of all, due to the numerous devices on which the TCAS system relies and the complex correlation system, the composition of the TCAS system and the common failures of the supporting equipment were introduced. Secondly, based on the principle of TCAS, the mechanism of TCAS failure caused by transponders, inertial navigation and atmospheric data datums was analyzed. Thirdly, an explanation was given on whether the failure of TCAS can continue to implement RVSM, pointing out that TCAS is not a basic device in reduced vertical separation minimum(RVSM) airspace, and it is necessary to distinguish the causes of TCAS failure. Finally, the TCAS failure and the associated equipment failure were analyzed from three stages, and the signage solutions of AP / FD TCAS mode loss, navigation TCAS failure, TCAS failure after taxi out and after take-off and associated system failure were analyzed.