Amid escalating geopolitical conflicts and extreme weather, the transformation to a renewable‐dominated power system faces increasing supply security risks. The current literature lacks comprehensive multidimensional assessment index systems to quantify these risks. This paper first identifies the key factors influencing supply security in renewable‐dominated power system. Subsequently, it constructs a supply security risk assessment model based on a comprehensive evaluation method combining the analytic hierarchy process method and the entropy weight method. The effectiveness of the approach is then evaluated through simulations of supply security risks across 11 provinces, including Inner Mongolia and Sichuan. Finally, the analysis of supply security risks in the Sichuan power system is conducted at different stages, focusing on changes in supply security risk levels and risk transmission mechanisms during Sichuan power crunch events in 2022. The results show that during Sichuan's 2022 high‐temperature power rationing event, the comprehensive supply security risk score of the Sichuan power system dropped from 88.7 to 76.91. This decline was mainly due to a significant reduction in hydropower output, which lowered scores of the source‐side risk and consequently increased the system's supply security risk.