Flexible resources (FRs) in power systems serve as a critical enabler for integration of high share renewable energy. Conventional planning practices mainly focus on supply-side FRs while neglecting demand-side flexibility potential

which challenges the coordination of multi-source flexibility contributions. This paper presents a comprehensive flexibility evaluation framework integrating both supply-and demand-side resources. First

operational models characterize supply-side FRs (thermal

hydro and pumped storage) and demand-side FRs-including centralized air-conditioning systems with cold storage (CAC-CS)

plug-in electric vehicles (PEVs)

distributed battery storage (DBS)

and time-shiftable appliances (TSA)-with parameters extracted from multi-source datasets. Second

a tri-dimensional flexibility metric system encompassing energy shifting

power balancing

and ramp rate is proposed

anchored by baseline net load. Third

a case study of a provincial power system reveals three key trends as the renewable penetration ratio (REPR) rises to 50%: 1) increasing system flexibility requirements dominated by power surplus and upward net load ramping; 2) shifting roles of coal plants toward deficit injection (surplus metrics dropping by >80%) and enhanced pumped storage utilization (4-10 times rise in surplus and ramping metrics); 3) PEV and CAC-CS account for approximately 50% energy shifting flexibility in each season and show similar variation trends in metrics to pumped storage. The framework enables coordinated source-load flexibility planning

demonstrating demand-side FRs' capacity to offset conventional flexibility limitations and support renewable integration targets.