The concave building structure (CBS) featured with a partial sunken façade is prevalent in the high-rise residence and comprehensive buildings. The building is a 3/4 enclosed longitudinal shaft that creates a new channel for longitudinal fire propagation. The objective of the paper is to reveal the dynamic mechanism of fire propagation in CBS. A small-scale experiment and Fire Dynamics Simulator (FDS) were conducted to investigate the fire behaviour. The results show that CBS could enhance the longitudinal propagation velocity of the flame and smoke with a slender profile. It could reduce the heat loss and create a higher space temperature than no-concave building structure (NBS). The maximum temperature and smoke concentration extended from the external facade centre to two flanks with the building height increase. Raising the structure factor extended the flame length from 7.0 m to 8.6 m. It exhibited a typical three stages of entrainment velocity growth: rapid growth, medium growth and slow growth. The flame transferred from the external façade to the lateral façade when the heat release rate was 8–10 MW. The research is essential to distinguish the hazardous region and provide a basis for architectural design and personal safe evacuation in the CBS fire scenario.
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