Based on the real-time digital simulation system RT-LAB, this paper establishes a typical mathematical simulation model of loop flexible DC distribution. The model includes photovoltaic, lithium batteries and AC/DC load. Simulation analysis of different running conditions transient response tests, such as start-stop control, command control, is made. RT-LAB significantly enhances flexible DC distribution simulation timeliness and practicability. Start-up logic design, soft-switching and progressive power upgrade method effectively reduce starting current and access operation over-voltage. It is proved that the model works well to meet the operational requirements, and the study about start-stop control and operation conditions can provide reference for further engineering construction.