Modal analysis conducted in the continuous frequency domain can identify the dominant nodes that influence the weakly damped modes in a system. However, as the number of converters increases, the system exhibits high-order dynamic characteristics, posing significant challenges to modal identification and quantitative analysis. To address this, a stability quantitative evaluation method based on a discrete state-space model is proposed. First, referring to the electromagnetic transient simulation modeling principles, an equivalent circuit model in the discrete domain is established. Numerical computation and modeling analysis are then carried out by computers, effectively avoiding the computational burden associated with symbolic variable operations in continuous frequency domain analysis. Second, the system modes are solved from the discrete-time model derived from the equivalent circuit. Node participation factor analysis is performed according to the frequency domain model of the discrete node admittance to identify the weak points of the system. Finally, combined with the chain rule, the sensitivity of system modes to converter parameters is quantified, enabling targeted parameter optimization to improve system performance. The study demonstrates that this analysis framework provides an efficient and reliable approach for stability evaluation of complex systems.