Visualizing blood flow velocity distribution is essential for comprehending the pathogenesis of various diseases and facilitating early diagnosis and treatment. Current hemodynamic studies utilizing optical coherence tomography (OCT) primarily rely on Doppler OCT (D-OCT) and OCT Angiography (OCTA), which measure axial blood vessel velocity or visualize the vascular architecture, respectively. However, these techniques have limitations in accurately quantifying the absolute velocity of red blood cells (RBCs). This study presents a novel method based on microsphere tracking, which enables precise quantification of absolute blood flow velocity along a blood vessel. In phantom experiments, freshly harvested blood mixed with microspheres was infused into a cellulose tube to simulate a single blood vessel. Experimental results, demonstrating an error margin of less than 10%, validated the effectiveness of this method. Blood flow velocities ranging from 0.472mm/s to 18.9mm/s were accurately measured. A preliminary in vivo examination of rabbit ear vessels was conducted, further validating the reliability of this method. This study presents a potential method for specific disease diagnosis by detecting targeted vessel flow velocity variations using swept-source optical coherence tomography (SS-OCT) combined with microsphere tracking.