Applying the PowerFLOW software using the Lattice Boltzmann Method (LBM), and combining with the very Large Eddy Simulation Method (VLES), the unsteady flow field of the MIRA notchback model was solved to study the structure and flow characteristics of the external flow field. By analyzing the process of the flow from the A-pillar along the roof to the C-pillar and the rear of the flow field, the structure and flow mechanism of the C-pillar vortex, D-pillar vortex, and partial separation vortex were explored. The transient flow field was analyzed to explore more precise random flow characteristics. In the time domain flow field analysis part, the flow field structure of the wheel, rear wind window and rear part is complicated. The analysis results of the frequency domain further showed the vibration frequency and pulsation characteristics of the vortex. The vibration frequency of the rear part is 12Hz, and the vibration frequency of the side window, top of the engine compartment, roof, and side of the body is 23Hz. In addition, the formation mechanism of the vibration frequency was explored. By analyzing the pressure pulsation, it is found that large vibration energy occurs around the chassis, rear part of the vehicle body, and rear wheel area. It is concluded that the flow field structure in the above area is complex and it contributes a lot to the drag. When comparing the simulation results with the experimental results, the flow field structure is similar, and the number and position of the vortex cores are well consistent, which verifies the reliability of the simulation.