Abstract: The multi-functional station integrated by charging station and substation (we call it transformer-electric charging for short) has become an advanced mode. It can make full use of the space resources of existing substations and provide convenient services for electric vehicles. However, the electric vehicle-grid interaction technology will change the security constraints of the transformer-electric charging station. Thus, the traditional substation load margin assessment method is no longer applicable. Aiming at this problem, a system security model of the transformer-electric charging station considering vehicle-grid interaction is firstly constructed. Then, the dimension reduction projection visualization method is used to display the model, and the assessment of load security margin is further proposed. Finally, the effectiveness of the method is verified by an actual power grid example. And the conclusion shows that the method provides a theoretical basis for the security analysis of the transformer-electric charging station as well as the urban distribution network.