A bionic fractal tree-like photobioreactor (PBR) with multiscale bubbles was studied to help intensify gas–liquid mass transfer and the microalgal photosynthesis. A numerical simulation and several experiments were conducted to study the hydrodynamic characteristics of the microbubbles (MBs) and large bubbles (LBs) in the tree-like PBR. The characteristics studied included the bubble size distribution, flow pattern, gas holdup, gas–liquid slip ratio, and turbulence. The good flow characteristic of the MBs tended to generate a homogeneous dispersed flow pattern, which intensified the synergistic effect of flow and light fields based on the long residence time, large gas–liquid contact area, and even multiphase mixing in the large-area illuminated fractal reactor. As a result, the photosynthetic capacity and microalgal activity were enhanced by combining the tree-like PBR with MBs, providing a novel valuable technical solution for improving the multiphase photoreaction.