Understanding the complex fluid flow in the alveoli is of great significance for studying the transport and deposition of fine particles in the deep lung. In this study, we developed an experimental platform to study detailed acinar flow through precisely controlling the flow parameters in a single microfluidic alveolar chip with rhythmic wall expansion. Numerical modelling was also carried out to study the flow parametrically. Detailed alveolar flow patterns at different generations were measured and compared with numerical simulation results. In spite of the low Re number, the alveolar flow is very complex and different flow patterns coexist in the alveolar tree. Stagnation saddle points in the alveolar flows were experimentally observed for the first time, suggesting the existence of complex chaotic flows in the alveoli which confirms the numerical predictions. This study provides valuable data for understanding the alveolar flow and the transport of micro- and nanoparticles in alveolar cells.