Under the conditions of liquid phase inlet Re ranging from 9,836 to 56,206 and gas fraction ? from 4.76% to 66.67%, gas-liquid two-phase bubbly flow was investigated in a static mixer with three twisted leaves (TKSM) with a diameter of 100mm and an aspect ratio of 1.5. A high-speed camera Revealer-2F04M with a resolution of 1,920 X 1,080 pixels was used to capture the evolution of bubble groups at the different axial windows of mixer elements. The results show that the flow pattern in the TKSM is still in bubbly flow at the flow rate of continuous phase QL no more than 1.0 m3/h and gas fraction ? higher up to 54.55%-66.67%. The Sauter mean diameter d32 of bubble groups gradually decreased with the increase of the mixing elements number. With the given liquid flow rate QL≤1.0m3/h, the Sauter mean diameter d32 firstly decreased and then increased with the increase of gas flow rate. The local minimum of d32 was obtained at QG=0.72m3/h and 84.5% of the dB/D0 is in the range of 0.02-0.05. The relationship among Sauter mean diameter, the inner diameter and the non-dimensional residence time τ satisfies the correlation We0.35·d32/D0=0.026τ ·0.17.