Due to the demand of green and low-carbon development, China is vigorously promoting the construction of large-scale renewable energy bases. Renewable energy transmission system through voltage source converter-voltage direct current(VSC-HVDC )working as island mode is receiving more and more attention. For the system, renewable energy devices usually adopt grid-following control, and the VSC-HVDC adopts grid-forming control. However, the synchronous stability mechanism for such a high-proportion or even 100% power electronic system is still unclear. Focusing on the role of phase-locked loops and virtual synchronous generator loops in the synchronization, a synchronization model for grid-following and grid-forming devices is established. Then, the small signal stability of the system is analyzed, and the operating area that ensures stable operation is derived. The operating states with instability risks are respectively proposed both for renewable energy and VSC-HVDC devices. Finally, the conclusions are verified in PSCAD simulation.