<p>Ag paste has gained considerable attention due to its excellent thermal conductivity and high service temperature. During the sintering process, densification is achieved through neck formation between Ag particles, and a porous structure is inevitably formed. Although sintered Ag paste exhibits high thermal conductivity, its performance remains lower than that of bulk Ag due to the presence of pores. Therefore, introducing doped diamond into Ag paste has been proposed to enhance the effective thermal conductivity of Ag paste. However, accurately predicting and optimizing the effective thermal conductivity of diamond-doped Ag paste remains a major challenge, primarily due to the presence of pores and the complex nature of interfacial thermal resistance. Therefore, based on previous studies on effective medium theory, the influence of pores on the thermal conductivity of composites is considered. The thermal conductivity of three-phase composites composed of pores, doped materials, and matrix is simulated in this paper. Ag-based composites doped with 5 and 15 vol% diamond were fabricated, and the thermal conductivity of diamond-doped Ag paste was measured. The error between the experimental value and the predicted value was 2.4 and 4.1%, respectively. The error significantly lower than the 6 ~ 20% prediction errors commonly reported for sintered Ag paste and composite thermal-conductivity models in previous studies. The model covers the influence of pore shape and porosity, doping particle and matrix type, doping particle size and concentration, coating type and thickness on thermal conductivity, and achieves good consistency at low doping concentration, which can be used to predict the thermal conductivity of three-phase composites at low doping concentration.</p>