Powder metallurgy-hot isostatic pressing (PM-HIP) has emerged as an important technology for near-net-shape fabrication of complex, high-integrity components, particularly for demanding applications in aerospace and power generation. Herein, using simple cylindrical specimens and a complex fan impeller, the influence of capsule wall thickness on the microstructure and mechanical properties of PM-HIPed Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy was systematically investigated. Additionally, the service performance of the PM-HIPed fan impeller was assessed under extreme operational conditions via spin testing conducted at room temperature (25 °C) and elevated temperature (300 °C). Results show that, under HIP conditions of 940 ℃, 120 MPa, and 3 h, capsule wall thickness variations of 5–40 mm had a negligible effect on the microstructure or tensile properties of the PM-HIPed alloy formed within these cylindrical capsules. Similarly, the complex fan impeller, manufactured under comparable HIP conditions, exhibited excellent microstructural uniformity throughout its intricate form. During spin testing, the impeller demonstrated remarkable stability at 300 °C, successfully achieving a stable rotational speed of 35746 r/min. At 42494 r/min, however, the impeller began to experience significant vibration, culminating in a loss of structural stability. To elucidate the mechanism of structural stability loss, finite element simulations were performed, complemented by transmission electron microscopy (TEM) analysis of the stability loss region. These investigations highlighted significant stress concentration at the fan impeller–shaft connection region as the primary initiator of the stability loss. TEM observations provided micro-scale evidence that the concentrated stress likely promoted extensive dislocation activity, with dislocations shearing through α₂-phase particles. This process facilitated planar slip behavior, intensifying localized plastic deformation within the shaft connection region and ultimately causing instability at ultrahigh rotational speeds. Furthermore, based on simulation results and interrupted experiments, a method for designing capsule wall thickness using threshold control of stress and temperature was proposed and successfully validated using cylindrical capsules.