Grid forming (GFM) voltage source converters (VSC) based on the power synchronization control (PSC) are used to support the grid by emulating the power angle characteristics of synchronous machines. However, they face oscillation risks under wide range of short-circuit ratio (SCR) conditions. To analyze the stability of GFM converters under wide range SCR conditions, this paper establishes a small-signal PSC-VSC model, and the transfer functions of active and reactive power are derived. Based on this, the small-signal instability mechanism of GFM converters under wide range of SCR conditions is revealed. The results show that PSC-VSC faces small-signal instability risks in strong grid conditions. Subsequently, by limiting the gain of the active power transfer function, the power self-synchronization control (PSSC) based VSC control strategy is proposed to enhance the stability of GFM converters in strong grid conditions. Furthermore, the stability of PSSC-based GFM converters under grid faults is analyzed. Finally, the correctness and effectiveness of the proposed analysis method are verified through hardware-in-the-loop real-time simulation.