【Objective】 Natural soil typically consists of a variety of colloids and multiple electrolytes coexisting simultaneously. However, existing studies on soil colloid coagulation have mostly been conducted under conditions involving a single electrolyte and a single colloid. In this study, the coagulation dynamics of a mixed colloid system comprising montmorillonite and humic acid under mixed electrolyte conditions were systematically investigated attempting to explain the coagulation effect and mechanism of mixed electrolyte on the mineral-organic mixture. 【Method】 Using dynamic laser scattering (DLS) technique, the coagulation kinetics of montmorillonite colloids and montmorillonite-humic acid mixed colloids under the influence of mixed electrolytes were studied by considering key parameters such as the average coagulation rate, critical coagulation concentration (CCC), and coagulation activation energy. 【Result】 The results revealed the following key findings: (1) Under mixed electrolyte conditions, whether involving a single colloid or a mixed colloids, there was only one CCC. This indicates that under mixed electrolyte conditions, both the two cations in the mixed electrolyte system play a cooperative role in the coagulation of the colloid. Although both cations collaboratively influenced colloid coagulation, the analysis of colloid coagulation rate, CCC, and coagulation activation energy revealed that the cation with stronger competitive adsorption ability played a decisive role. For example, in the 99% Mont (Montmorillonite) + 1% HA(Humic acid)mixed colloids system, the CCC (97.41 mmol·L^–1) of the Na⁺ + K⁺ mixed system closely resembled the CCC (94.91 mmol·L^–1) of K⁺ alone system, indicating that the role of Na⁺ in the coagulation of this mixed colloid was almost negligible. This implies that in the Na⁺ + K⁺ mixed system, K⁺ plays a decisive role in colloid coagulation. (2) Increasing the content of humic acid significantly enhanced the stability of the mixed colloid, resulting in the requirement of a higher electrolyte concentration to induce colloid coagulation. This effect can be attributed to the fact that the addition of humic acid increased the surface charge density of the organic/inorganic composite colloidal particles, thereby strengthening the electrostatic repulsion between particles. 【Conclusion】 The scientific findings of this study not only provide guiding significance for further unraveling the formation mechanism of soil organic-inorganic complexes, but also shed light on the cooperative role of cations in colloidal coagulation under mixed electrolyte conditions. The results underscore the decisive role of the cation with stronger competitive adsorption ability in colloid coagulation. Furthermore, the study reveals that increasing the content of humic acid significantly enhanced the stability of the mixed colloid, necessitating a higher electrolyte concentration to induce colloid coagulation. These insights contribute to the understanding of the complex interplay between organic and inorganic components in soil, paving the way for future research in this field.