[en] Highlights: • A framework integrating novel model, co-optimization, and operation evaluation. • The small signal model is competitive in simulation efficiency and applicability. • The adaptable co-optimization strategy could coordinate the stability-tracking conflict. • The 12 scenarios verify the competitive advantage of proposed method. The fast and stable regulation of pumped storage is a basic guarantee for supporting various scenarios of renewable energy system. The operator pursues sensitive tracking performance, while underestimates the dynamic characteristics of hydraulic system and damping characteristics of pumped storage unit (PSU). These may aggravate the wear-tear of PSU operation, and decrease the frequency stability of power system. Therefore, a systematical study of improving overall regulation performance is conducted by applying PSU modeling, co-optimization and operation evaluation. First, the novel small signal model is proposed and the high-order hydraulic damping model is further derived. Apart from the ultra-low frequency oscillation mode, a new frequency oscillation mode caused by surge tanks is captured. Second, the co-optimization strategy is presented to coordinate the stability-tracking conflict. And then a comprehensive evaluation model, integrating 14 indicators is conducted to quantify the PSU performance and its contribution to power system, driving the favorable decision making of operators. Compared with the original scheme, the overall performance of PSU is improved by 20.76%, at the cost of 10.88% tracking capacity. The comparison of three measures including 12 scenarios verifies the competitive advantage of co-optimization strategy in multi-machine system. This paper supplies a novel tool for performance enhancement of PSU. It may have potential value in the stability of renewable energy systems with multiple hydropower units.