Abstract: The characteristics of bubble flow in gas-liquid two phase flow in the bio-doubling reactor under seven different inlet flow rates were investigated by using particle image velocimetry (PIV) technology. The bubble flow field in the two cross-sections of the reactor was decomposed further using proper orthogonal decomposition analysis for extracting the large scale structure of the flow. The analysis results show that the axial velocity decreases, the radial velocity increases and the Coanda effect is enhanced with the increase of the influent flow rate. As the influences of different inflow discharges on the bubble flow are different, so that there are differences between the velocity field and vorticity distribution of the bubble flow, and the vortex occurs in the bubble flow field and the bubble flow field produces vortex flow under a specific working condition. When the inflow discharge in the upper region(A) is 0, the highest energy content of the first order modes is about 7.2% of the total kinetic energy, and when the inflow discharge in the lower area(B) is 0.18 m³/h, the highest energy content of the first order modes is about 4.5% of the total kinetic energy. The proportion of the first order mode is not quite different in the A and B regions. There are obvious large scale structures in the low order characteristic modes, and the large scale flow structure is indirectly related to the liquid mixing behavior. When the inflow discharge is 0.90 m³/h, the area of two cross sections of the reactor is the most uniform and the gas phase dead zone is the least, and at this time, the flow regime is the best. Under such experimental conditions, when the inflow discharge is 0.90 m³/h, the overall bubble flow regime of the reactor is the best.