In 100% power electronics-based power system, small-signal synchronous instability can be trigged by interaction among distributed grid-forming converters (GFMCs). A novel small-signal synchronous stability analysis method is proposed. Firstly, a generic small-signal model for 100% power electronics-based power system is established. Then, a framework based on a dominated synchronization control loop is proposed, which can explicitly identify two main interaction paths stemming from voltage control and reactive power-voltage control. Furthermore, the two interaction paths have been simplified to first-order transfer functions through model reduction based on dominated modes. By integrating the selected synchronization control loop, a reduced second-order model that can provide clear physical insight into small-signal synchronous stability is derived. Finally, experimental results from an RT-LAB hardware-in-the-loop platform confirm the effectiveness of the proposed method.