The underground interconnected infrastructures are under development in the metropolitan areas, and simultaneously, also raise many safety issues, especially with regards to the smoke control once a fire occurred. Owing to the interconnected structure, the smoke spread in underground interconnected infrastructures can be more complex than that in “normal” tunnels, where the smoke spread is commonly simplified as one-dimensional spreading. In this study, the properties of maximum temperature, temperature decay, and smoke spread in a longitudinally ventilated underground interconnected infrastructure were studied. Experiments were conducted in a reduced-scale model which was divided into the tunnel part A and part B, and the scenario of a fire occurring at the junction point was mainly considered. For such a scenario, it is found that the temperature decay depends on the ventilated velocities of both upstream and connected tunnel sections. The spreading of the smoke in the upstream tunnel section depends on the ventilated velocity of the upstream tunnel section and can be well described by previous back-layering correlation. Whereas for the spreading of smoke in the connected tunnel section, both the ventilated velocities in the upstream and connected tunnel sections show a distinct impact. The estimation of whether the smoke spreads to the connected tunnel is established by comparing the buoyance force of the smoke and the inertia force of the ventilated flow. It is indicated that the smoke flow will be less inclined to spread to the connected tunnel under the scenarios of weak buoyancy force and large ventilated velocity. The findings of this work may contribute to an improved understanding of thermal-driven smoke propagation in underground interconnected infrastructures.