Abstract: Cab signal code missing is a common fault in daily operation of ATP onboard equipment, which can directly trigger train braking and affect the efficiency of train operation. The occurrence of cab signal code missing in scenarios of track circuit special design often occurs during the joint testing and commissioning stage before the opening of new lines or after line renovation. Since their causes are complex and varied, and it is necessary to combine the decoding logic of onboard ATP equipment and the specific design of ground track circuits for comprehensive train-ground analysis. The article conducts an in-depth analysis of the causes of two incidences of cab signal code missing that occurred in special design scenarios of track circuits based on the data transmission process of track circuits, the track circuit signal decoding process and the judgment logic of cab signal code missing of on-board ATP equipment. Combined with the processing logic of ATP on-board equipment and the actual situation on site, effective and feasible corrective actions are proposed from the aspects of difficulty, safety, and economic cost of on-site implementation, and verified via on-site testing, which can provide reference for the daily operation and maintenance of ATP on-board equipment.