Abstract: Abstract: Algae as potential resources, has attracted increasing interest and attention from many fields such as energy, medicament, food, feed, and environment. However design and optimization of photo-bioreactor for algae production remains a bottleneck in the development of microalgae culturing industry. Recently, flat photo-bioreactor is improved by changing the double-flat into multistage structure. In this study, in order to further increase mass transfer and mixing properties of microalgae photo-bioreactor, a multistage intake structure was fixed into this multistage flat photo-bioreactor. Moreover, for the purpose of exploring more mass transfer and mixing details of the three photo-bioreactors, the ordinary double-flat photo-bioreactor, multistage flat photo-bioreactor, and multistage intake photo-bioreactor were constructed physically and numerically, and their computational fluids dynamics (CFD) simulations were carried out. Gas holdups and mass transfer coefficients were measured in physical multistage intake photo-bioreactor and compared with simulated results to verify reliability of the applied CFD simulating method. Then speed cloud, mean liquid velocity, dead zone, turbulent kinetic energy, turbulent kinetic energy dissipation rate, gas holdup, mass transfer coefficient were used to analyze mass transfer and mixing performances of the three reactors. Results of gas holdup and mass transfer coefficient showed that the CFD simulation agreed well with experimental results, which indicated that the CFD simulation method in this work was reliable. Results of speed clouds showed that the multistage-intake structure caused significant emergence of circulating current in every stage, which tended to form in the multistage flat reactor but failed finally. Liquid velocities of these currents increased from bottom to top of the multistage-intake reactor. Compared with the other two photo-bioreactors, the currents in the multistage-intake led to increase in indexes (e.g. mean liquid velocity, dead zone, turbulent kinetic energy, turbulent kinetic energy dissipation rate, gas holdup, and mass transfer coefficient) of the multistage-intake photo-bioreactor. In point of mixing properties, differences of some indexes (including mean liquid velocity, turbulent kinetic energy, and turbulent kinetic energy dissipation rate) between the multistage intake reactor and the other two reactors became larger when aeration rate (volume of air intake per minutes over the volume of a container) was greater than 0.4. Meanwhile, difference of dead zone ratio was the highest when aeration rate was 0.6. At the point of mass transfer properties, differences of gas holdups between multistage intake reactor and the other two reactors increased when the aeration rate increased to 0.8 and decreased after 0.8. At 0.8, the gas holdup of the multistage intake photo-bioreactor was 52.63% and 39.11% higher than those of ordinary photo-bioreactor and multistage flat photo-bioreactor, respectively. As the most direct index, mass transfer coefficient of the multistage-intake photo-bioreactor was increased by 36.16% and 11.27% comparing to ordinary photo-bioreactor and multistage flat photo-bioreactor, respectively. All the above results indicated that, multistage flat photo-bioreactor performed better than double-flat photo-bioreactor. However the newly-built multistage intake photo-bioreactor gave the best performance in both mixing and mass transfer properties among the three types of reactors, especially for the aeration rate ranging from 0.4 to 0.8, the suitable aeration rate for microalgae culturing. The multistage intake structure put forward in this paper is of great assistance to design and establishing optimal flat photo-bioreactor.