Complex thermal behaviour during fabrication plays an import role in the geometrical formation and mechanical properties of Ti6Al4V components manufactured using Wire Arc Additive Manufacturing (WAAM) technology. In this study, through in-situ temperature measurement, the heat accumulation and thermal behaviour during the gas tungsten wire arc additive manufacturing (GT-WAAM) process are presented. The effects of heat accumulation on microstructure and mechanical properties of additively manufactured Ti6Al4V parts were studied by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and standard tensile tests, aiming to explore the feasibility of fabricating Ti6Al4V parts by GT-WAAM using localized gas shielding. The results show that due to the influences of thermal accumulation, the layer’s surface oxidation, microstructural evolution, grain size, and crystalline phase vary along the building direction of the as-fabricated wall, which creates variations in mechanical properties and fracture features. It has also been found that it is necessary to maintain the process interpass temperature below 200 °C to ensure an acceptable quality of Ti6Al4V part fabricated using only localized gas shielding in an otherwise open atmosphere. This research provides a better understanding of the effects of heat accumulation on targeted deposition properties during the WAAM process, which will benefit future process control, improvement, and optimization.