<p>The thermodynamic properties of gassy soil are crucial for improving the construction process of tunnels using the artificial freezing technology in shallow gas distribution zones of the Quaternary system. Various initial saturation levels of soft soil specimens containing large air bubbles were prepared through indoor reshaping, and the effective thermal conductivity (ETC) of gassy soil was measured using the transient linear heat source method. The relationship between saturation and ETC was investigated. Three classic unsaturated soil ETC models were employed for comparative prediction analysis, and fractal geometry theory was introduced to propose an ETC fractal model for soft soil containing large bubbles. The results show that the ETC of soft soil containing large bubbles decreases linearly with the increase in bubble content. The classical ETC models for unsaturated soils are unsuitable for predicting ETC in gassy soils, as they tend to overestimate the thermal conductivity. The main reason is that the size of bubbles inside the gassy soft soil exceeds that of the matrix pores, and the unique microstructure morphology leads to a smaller and more tortuous heat transfer path. The established ETC fractal model for soft soil containing large bubbles accounts for the effect of increased pore tortuosity caused by the presence of bubbles and effectively describes the evolution of ETC. Combined with experimental results, the accuracy of the model has been verified, providing a reference for frozen construction projects affected by the Quaternary shallow gas geology.</p>