Given that the departure capacity of depots has gradually hindered the improvement of the operation level of the main line during morning peak hours, some cities have begun to explore the construction of ATC depots. However, existing calculation methods for vehicle sending and receiving capacity cannot be applied to ATC depots. The throat area capacity calculation method of ATC depots is investigated to compensate for the above shortcomings. First, under the ATC lightsoff mode, the minimum total train departure time is obtained by comparing the total train departure times under different train departure sequences. Second, the maximum throat area passing capacity of the ATC depot is calculated. Finally, using the Guangzhou Luogang depot as a case study, a validation study of the effectiveness of the calculation model of the throat area passing capacity under the ATC lightsoff mode is carried out. The results show that the passing capacities of the throat area under the ATC lightsoff, ATC lighting, train adjustment combination, and train approach modes are 28, 17, 13, and 11 trains/h, respectively; the ATC lightsoff mode has a greater capacity than the other three modes and can better meet the operational demand of the mainline during morning peak hours. The results of this study can serve as a reference for evaluating depot design schemes.