Counting the number of red blood cells (RBCs) in blood samples is a common clinical diagnostic procedure, but conventional methods are unable to provide the size and other physical properties of RBCs at the same time. In this work, we explore photoacoustic (PA) detection as a rapid label-free and noninvasive analysis technique that can potentially be used for single RBC characterization based on their photoabsorption properties. We have demonstrated an on-chip PA flow cytometry system using a simple microfluidic chip combined with a PA imaging system to count and characterize up to similar to 60 RBCs per second. Compared with existing microfluidic-based RBC analysis methods, which typically use camera-captured image sequences to characterize cell morphology and deformation, the PA method discussed here requires only the processing of one-dimensional time-series data instead of two- or three-dimensional time-series data acquired by computer vision methods. Therefore, the PA method will have significantly lower computational requirements when large numbers of RBCs are to be analyzed. Moreover, we have demonstrated that the PA signals of RBCs flowing in a microfluidic device could be directly used to acquire the osmolarity conditions (in the range of 124 to 497 mOsm L-1) of the medium surrounding the RBCs. This finding suggests a potential extension of applicability to blood tests via PA-based biomedical detection.