As a supersonic transportation system, the flow around a vehicle in an Evacuated Tube Transportation (ETT) system will evolve through series of flow structures during acceleration. The occurrence of choked flow and shock wave will especially lead to the drastic change in flow field structures. In this study, based on the one-dimensional inviscid flow assumption, a theoretical model is established to quantitatively describe the formation time, formation location of choked flow, and Frontal Normal Shock Wave (FNSW), as well as the distance of the disturbed flow field region ahead of the vehicle in the ETT system. It is found out that the formation time of FNSW as well as the initial distance between the vehicle head and FNSW is linearly proportional to the blockage ratio while linearly inverse proportional to the acceleration rate of the vehicle. An experimentally verified numerical model is also established with an overset mesh technique to investigate the flow field evolution for vehicles accelerating in the ETT system. The results from numerical analysis agree well with the theoretical model. Meanwhile, five typical flow field structures are summarized for a vehicle accelerating from a stationary state to supersonic state in the ETT system. The applicability of the wind tunnel method and overset mesh technique in numerical simulation of the ETT system is systematically discussed. The influence of occurrence and dissipation of choked flow and shock wave on the vehicle's aerodynamic drag profile are then analyzed quantitatively.