Titanium alloys have the advantages of high specific strength, fatigue resistance, and corrosion resistance. Also, they are widely used in the aviation, aerospace, weapons, petroleum, and chemical industries and other fields. The use of large-scale and integrated aviation forgings, which are an important development in titanium alloy manufacturing technology, can increase the service life, safety and reliability of aircraft structures and engines, and simultaneously reduce their structural weight and shorten their manufacturing cycle. However, problems such as a decline in mechanical properties and the presence of abnormal low-magnification structures due to the strong β phase texture have gradually been revealed. For example, large-size near-β titanium alloy bars often have the problem of coarse and uneven macrostructures, and the center layer of these bars tend to form a strong {100} β phase texture. These defects are easily inherited in the forgings, which adversely affect their performance and threaten their safe use. In this work, 300 mm diameter TC18 titanium alloy bars were used as the research material. The SEM and EBSD techniques were used to study the microstructure and texture characteristics of the β phase after thermal deformation, respectively. This work compared the influence of the thermal deformation parameters (compression/stretching, deformation temperature, reduction, strain rate, and holding time) on the evolution of the β phase microstructure and texture in the TC18 titanium alloy. Also, the deformation, dynamic recovery, dynamic recrystallization, and grain growth behavior of the β phase were investigated. The results showed that when the TC18 titanium alloy was compressed and stretched in the two-phase region, the β phase was mainly dynamic recovery. After thermal compression, the {100} and the {111} textures were mainly formed, while after thermal stretching, the {110} texture was mainly formed. When it was compressed in the β phase region, as the deformation temperature increased, the reduction increased, the strain rate decreased, the strength of the {100} texture increased and the {111} texture decreased. When it was compressed in the two-phase region, as the deformation temperature increased and the reduction increased, the strength of the {100} texture increased and the {111} texture decreased. When it was stretched in the two-phase region, as the reduction increased, the strength of the {110} texture gradually increased.