In high-penetration renewable energy multi-infeed systems (multi-infeed systems), large-scale integration of power electronic converters may lead to small-signal stability issues. Assessing system voltage support strength (system strength) is a critical tool for analyzing such problems. To address the limitations of existing researches that often neglect the reactive power support characteristics of converters and actual operating conditions, this paper proposes a small-signal stability analysis and system strength evaluation method for multi-infeed systems. First, a small-signal model of the multi-infeed system is established, incorporating both reactive power support characteristics and actual operating conditions. Second, an equivalent homogeneous multi-infeed system is constructed using the participation factor weighting method. Furthermore, based on modal decoupling theory, an equivalent single-infeed system is derived to characterize the small-signal stability of the multi-infeed system. Finally, a system strength evaluation metric accounting for reactive power support is proposed, and quantitative evaluation procedures are designed for both white-box models (with known control structures and parameters) and black-box models (with unknown control structures and parameters). Simulation results demonstrate that the proposed method can accurately evaluate system strength and exhibits good engineering applicability.